1) Clones are 1-day copy. You can adjust your strategy freely as long as you keep it in mind. Happily walk over water if you aren't going anywhere near water the next day. You can also shoot them to death as much as you like if you are going full stealth the next day etc. As in "their day 2 behavior (day+night) = your day 1 behavior during day", "their day 3 behavior (day+night) = your day 2 behavior during day".
2) Don't eat during day. I'm under the impression they run faster if they also eat. Oh, there's also poison which you can eat to slow them down? I know there's poisons and they must do SOMETHING right? I never really test this idea out apart from the one time I got the impression. So, I could be wrong.
3) When you've earn enough points, day will turn to night. Everytime the icy hologram copies you it seems to give a set amount of point (every action give different points but I have no idea how much). I usually just press R and shout softly afew times if I know I'm close to night. Killing clones award you the most amount of points. Just kill a bunch if you want to force night to come.
3+)Update: Based on user 'Rise', getting noticed works too and it might be faster than killing. Now that I think about it, this method is also much safer if done properly. By personal exp, I turned day to night more often while killing but I never did proper testing to be able to say for sure.... Plus, I played the game truly-stealth-style so I was hardly ever noticed by anyone. One thing to note however is that stealth killing a clone means you're also being noticed by it and you are doing a unique action of "takedown". All these makes killing very points intensive.
4) Clones who die 7 secs (during night) prior to day time will not respawn. The transition is indicated by some sound effect.
5) With (4) in mind, just run around and lure a whole bunch of them (this is why you shouldn't eat during day cos they'll catch up easily) and use riot gun to gather them alive being knocked down. When the day time sound effect begins, just shoot them dead. You'll basically get free roam the next day with no enemies.
6) With (4) in mind, DO NOT kill them too earlier. They WILL spawn right next to you if you're standing at the wrong place and you will be in big trouble.
7) Each transition to day time will render the clones useless for about 3 secs. You can do risky stuff or takedown one of them during this time or just go and hide.
8) Struggle with 3 clones is instant death but struggle with 2 is great since it means they will both be knocked down. Do it when there's only 2 enemies since a 3rd one will still get you if you do this.
9) Elevators are your best friend. Just spam the E button and you are immune to death. Yes, even if there were enemies on both ends. It also advances day to night.
10) Corruption is your best friend too.. You can restart overpowered clones to nothing. This work if you've done nothing for them to learn during the current new day you are in. Doing this at night is a waste since you've already survived daytime. Its useful but I've never had to resort to this and I dont see the corruptions alot.
11) Just to add, its easier to line enemies up around a corner since they have to go around it one by one. Since we can kill them in straight lines, its definitely worth considering when you plan your move.
12) Also, crouching made it easier for me to connect clones in a straight line. Its worth considering but its also kinda risky since they're all running towards you.
13) When its yellow, that means a clone is about to see/notice you.
14) When its red, that means it sees you.
15) When the radar indicates orange it means they can't see you and are only trying to find you.
16) Running around something that blocks view can make the clones lose sight of you. Even the really short ones. But some pillars have big gaps in them and aren't good for blocking sight.
17) Take note of where you are when radar is no longer red since that will be where the clone will be standing around feeling confused. From there you can either run away or do a takedown.
18) If there isn't any pillar around to hide with, being out in the open, you can also simply push it and then run slightly further. They usually will not follow because they've lost you. Strictly speaking, the last time it saw you was the moment you pushed it. Just make sure you are out of their sight when they stand up.
19) Note that you can hold F even before you're close enough for a silent takedown. This allows you to start the takedown immediately when you're in range.
20) Apart from (17), shouts are also useful when you are next to something that blocks view like a pillar etc. Lure enemy to you while you merry-go-round it for a silent takedown.
21) You can push them to their death as such : https://youtu.be/35a6BNRP5zM
22) You can shout them to their death as such : https://youtu.be/BHUxSSevKKE
23) You can explode them to their death as such : https://youtu.be/fIQ8VpTSVZM

For completionist like myself I was very annoyed about not finding everything before proceeding.. This is even worse when I realised I wasted time looking around when they're actually LOCATED AFTER THE STAGE IS COMPLETED. Below are the times when it happens:

Momentum: 1 Cell.

Baths: 1 Cell.
Sympathy: 1 Voice, 1 Cell.
Rise: 1 Cell
Peace: 2 Voice.

Rings: 1 Voice, 1 Cell.

Since there's no map its better to just have a video of it and you can watch for afew seconds to get your bearings. I'll only do orbs for chapter 2 since they're all revealed in the rest of the game. I'll point you towards the appropriate videos (some of them are spread out into 2 videos) and the timestamp will be in their description. After-stage collectibles will have the words (After) where timestamp is indicated.

Discovery AND Evolution:
https://youtu.be/guyVsfBbBaI
Infinity:
https://youtu.be/F0MGwmTXJ5M
Compulsion AND Momentum AND Entrance:
https://youtu.be/24LxkoiZsBI
https://youtu.be/Uq4RCsUyw0Y

Baths AND Sympathy:
https://youtu.be/mMEWlmyCMVY
Rise AND Peace:
https://youtu.be/f4PW39htFcE
Purity:
https://youtu.be/dQBq4YUc2Yo

Stairs AND Ballroom:
https://youtu.be/a6b4Oi-PQY4
Lounge AND Vitality:
https://youtu.be/yg8iGEzwdZQ
Vitality:
https://youtu.be/OtWFKWB7b0M
Banquet AND Bridges:
https://youtu.be/eIjRWV8avq4
Babel:
https://youtu.be/a5z4GKyry0Y

Antechamber AND Rings:
https://youtu.be/9u1mo9PF8gI
Alone AND Tombs:
https://youtu.be/IIOqWN5JE1U
Abyss:
https://youtu.be/YRkPVnPJ18U
https://youtu.be/D6y64Lqnebw

Reflection:
https://youtu.be/Yml8Fl6jQ9A
Order:
https://youtu.be/5lhun2L8HCQ
https://youtu.be/AfcvbZyXO5g
Reflection II AND New Gods:
https://youtu.be/l1FKEL13bQY
Ascendency:
https://youtu.be/fVs2O5aa_8A
Beyond:
https://youtu.be/Dbx_gtrGC0Q

Basically, the palace is a place where immortality has been achieved. The way it is done is poetically descripted by the voices. Steps as follows: (1) Abandon your flesh (its hinted that you actually need to be dying or dead) (2) set your potential free of time (become the cube) (3+4) overcome the deformed clones (once trials started you can only succeed or die trying) (5) you win, face your soul, sacrifice yourself and become immortal. It also says "all of them you will wear again" so I'm guessing En is also immortal whether literally or metaphorically. I don't know what the immortals are actually like but I'm guessing its a combine of the red cube person(s) combined with the victor. Maybe a guy with a girl soul or a guy with 2 souls.

There are 4 beds around the red cube machine so I'm guessing 1 victory grants 4 immortals.. So they wasted 3 slots in the game. I believe the guy woke up in an entirely different place than the red cube machine at the start. Mainly due to the birds chirping which is missing throughout the game. It makes sense. The 1st red cube machine serves as a door along with the game's ending where En walked into the light. They are both doors for participants' body and soul to attain immortality and go to where all the selfish jerks are. They only share immortality with those who fully entertained them... or passed the test. Depends on how you see it.

Other notes:
1)Push Era causing splinter of humanity and the extreme human constellations. Constellations not as in the stars but as in their knowledge. Humanity tech progression and philosophy is extremely diverse. Probably due to time dilation and availability of resource.

2) Foster is a master of manipulation. (It is also possible that he is just a super nice guy who is depressed due to being disillusioned over certain things.) Consider the following:
a) Foster was good at talking enough to talk London out of depression and made him his AI partner.
b) Consider the fact that London has spent a lot of time with Foster and he insists that he is a cold person with no shred of kindness. While En insisted Foster saved her and she is now obliged to revive him.
c) En ends up going to the palace and risked her life just so she can revive Foster. Due to guilt that came out of questionable circumstances as London had repeatedly pointed out. AND this is the exact thing she strong opposed and wanted nothing to do with it.
d) The lighter, that holds the coordinate to the palace, means a lot to Foster as he was described to have held it firmly in his hand looking at it and talks about it with a cold conviction.
e) Foster's supposedly kindness and sense of morality only emerged when En was brought next to a translator to be made into a red cube. It is likely he knew what was going on when he saw the machine and decided attack Gramps and started the events that led to the ending of the game.
f)It is possible that him being killed or seriously injured wasn't part of his plan. But even then it doesn't suggest ignorance/innocence/lack of manipulation on his part.
g) It is unlikely En, on her own, will know about the ligher's significance. It is very likely Foster told her about it and gave her the lighter. With this in mind, En still insisted that SHE took the lighter. London, a clear minded AI, insisted that it was Foster who gave it to her.
h) En honestly believed that SHE was the one doing manipulation as she said "He was good but I was better" and "He felt sorry for me because I was designed to be so.".

3) The palace is far away from Earth. We know that En is from the Earth as reference was made. Based on the conversation between En and London we know that they spoke for the 1st time since the event that caused Foster's death and that she was on stasis for a 100 yrs.

4) Gramps had no idea what is needed to be done.
a) En has repeatedly mentioned that Gramps, in the end, only had the words but not their meaning.
b) This becomes more and more obvious to En as she explores the palace and learns more about it.
c) Based on the poem, it is likely that a red cube will only be produce if the subject was dead or dying. En had mentioned numerous successful predecessors who were never seen again after translation attempts. Knowing that 1 red cube is sufficient, we can conclude that Gramps never managed to create a red cube of his own and ended up killing a lot of people for no reason due to his ignorance.

5) Its entirely possible that people who created the palace live on another planet altogether and the palace is just a seperate place. Consider the follow:
a) Even with exact coordinates London almost completely missed the palace.
b) The palace gave no gravitational pull. This suggests extreme cloaking and highly advanced tech.
c) The palace is basically a deathtrap.
d) It is very likely they've sent out numerous invitation to the palace as we know that humanity has spread out throughout the universe and the palace is simply too elaborate to be intended for only humans on Earth.
e) With (d) in mind, it is obviously unwise to reveal the locations of their home. Being invaded and outnumbered is a possibility.
f) The entire game is for gods of the palace's entertainment. Hint: Throne, lounges, theatre settings, banquets.

6) About Gramps:
a) Gramps knows how to fight aka badass. Kinda like Father Elijah. He died but killed Foster and gave En serious injuries.
b) Gramps isn't immortal. Hint: Gramps' words: "Why fight to get the best out of the little time you have on Earth when you can fight for so much more".
c) The religion has been around for centuries. Hint: En says "My life was decided centuries before I was born".
d) This religion follows the principles of Post-Scarcity Socialism. As hinted at by the points mentioned.

7) Foster as character design:
a) I feel that Foster was meant to give an interstella PI vibe due to his work nature and his lighter and London saying how he is disillusioned about life and relationships.
b) Foster is the most interesting character in the story despite his lack of actual presence. Not because we only hear bits about him but because we can't really tell what his intentions are. I believe he'll still be interesting even if he actively appears in the game.

8) London is an AI copy of an actual person in some sort of experiment to gain immortality. There was mention of lobotomy.
a) The use of the word lobotomy felt more surgical than it was programming and she could had used a more geek term.
b) He was capable of depression (compromising his work as he admitted) which he had to recover from after some counselling by Foster.
c) True emotion is a rather odd (and complex) thing to go out of your way to give a programme and it might give unforeseen problems which leads to (d) below.
d) Why is London even still around? People were paranoid about AI weren't they? Its risky and troublesome to modify and keep London around than say.. Just uninstall and replace.. UNLESS there are thigns you can't reproduce because it wasn't man-made. Then maybe we might not want to delete London out of existence just yet.
e) He's got a British accent. This could be man-made but from story writing perspective I'd like to think its their way to further emphaize he is more human than AI.
f) He wasn't exactly forced to respond to a specific owner or someone with authority/priviledge. Basically he had the free will to help or ignore people based which also makes him more human than AI.
g) With (f) in mind, London is obviously made as a work-assist AI and not a companionship AI. To give him freewill or emotion would be odd unless it was always there. Just think... siri.

h) From a story writing perspective, I think its obvious enough he was an AI. The details, about London, revealed in the game only made his identity as an AI ambiguous. Perhaps they are trying to explore the blur between machine and human since Foster was literally a red cube for a century. So its definitely fun to say that London on the other hand despite being nothing more than just codes all along is more human than any other human. But to say that London was never human is inconsistent in that there isn't any other example of AI that is intelligent or possesses emotions. So I took it as a hint that he wasn't always just an AI.

9) The palace essentially tested a person's soul/mind. Physique doesn't matter since all the clones are as strong and fast as you are. They even copy your behavior. En's training also says "Self-control so deep it could over come death itself".