Unless you have no experience with this type of games: Hard.
The game was originally designed with only one difficulty equivalent to Hard, and from my experience the game is balanced around that.

Tier 1. Available approx early act 2. Join as level 7.

• Sir Leodegrance Sage
  
• Sir Bedivere Vanguard
  
• Black Knight Champion
  
• White Knight Champion

Tier 2. Available approx mid act 2. Join as level 9

• Sir Lucan Sage
  
• Sir Geraint Marksman
  
• Sir Dagonet Arcanist
  
• Sir Damas Marksman

Tier 3. Available approx mid act 3. Join as level 15

• Sir Lancelot Champion
  
• Sir Galahad Defender
  
• Red Knight Defender
  
• Morgana LeFay Arcanist

Typically, you'd unlock 5 heroes through Morality Chart in a playthrough

• The Tier 3 ones in particular have pretty unique skills (as in access to skills from a different class).
• By definition, the Morality unlocked heroes have similar alignment to you/Mordred, thus increasing their loyalty.
  
• The quests to get them are always worth doing, for loot, XP and fun, even if you end up keeping them in the Aspirant slots.

If your active hero roster is full, the Aspirant slots in the Round table are a waiting list of unlocked but inactive heroes. There is no limit to the number of Aspirant heroes you can have.

Unfortunately you can move heroes freely from the Aspirant Slots to the active hero roster if you have free slots in the latter, BUT not the other way around: To free slots in the active roster you need to dismiss a hero permanently (or let a hero die, leave...).

And yes it sucks not being able to move heroes from active list to an aspirant slot, I hope that changes...

The only advantage is that heroes of certain morality won't particularly dislike you for having the opposite alignment to them. So no not really, focusing 100% on one alignment in each axis is what I would recommend.

At highest loyalty you get a bonus AP (Action point) and bonus damage, so definately worth it. Inversely you lose damage, AP and could lose the hero altogether at very low loyalty.



To increase loyalty:

• Give them Titles (see Round table), you'll unlock more titles as you progress in the main quest.
  
• Random events can also increase or decrease loyalty.
  
• Be aligned with them on the morality chart.
  
• Look at the traits of each hero, some have unique ways to gain/lose loyalty.
  
• There are trinkets items that give a few (+2 or +3) loyalty points.

No. The in-game logic is that you don't loot new weapons/armour but you loot and apply Sigils to your existing weapons/armour. And yes, they are left-handed and that can't be changed either.
(There is an alternative skin for Mordred, unlocked by subscribing to a newsletter, but that is the one exception, and may be unavailable by the time you're reading this)

Nominally 4 characters. There are many missions where a 5th will join you for a short duration, or for almost the whole mission.
For example the missions where you recruit new Heroes, you get your 4 original heroes + the 5th being the one you're recruiting.

• When selecting a melee opponent a red outline appears (areas where the enemy can move to and attack) and an orange one (where the enemy can move to but can't attack). Sometimes the best option is to step back from the threatened area and pass: Let the enemy use his AP to move and you have bonus APs for the next round, when they're close.
  
• With your melee heroes, close down on enemy ranged attackers: Enemy archers will have to shoot with a penalty or move and trigger an attack of opportunity.
  
• Inversely: kite the big melee enemies: They are usually slow and can be slowed down further with hexes, knockbacks and the like.
  
• Backstab enemies, but avoid getting backstabbed yourself: You can change orientation of your heroes at no AP cost. Ranged attacks can also "backstab".
  
• I also found Taunt a particularly useful skill if one of your heroes have it: Not only will the enemies try to attack your Defender rather then the squishy next to them, but they can trigger overwatches and attacks of opportunity as they try to close in on the Taunter.

Training grounds.
If your alpha team is injured or otherwise unavailable and the backup heroes are 3 levels below the only available quest, you could be in a situation where even reloading an old save can't save your run.
On a side note this building is also the one you need to refund skillpoints: It costs some gold and the hero will be unavailable for 1 week.

There isn't any!
During combat you'll find the only way to reliably restore health is a healing potion.

That said there are preventive ways to avoid losing health to begin with: Skills to increase armour temporarily, grant a temporary pool of bonus HP or even evading or absorbing damage altogether.

Between fights, campfires and the odd healing shrine is what you have to rely on. Use them strategically, don't necessary use them as you stumble upon them.

Action points (APs) come into two types Green APs that can be used for either movement or skills and Yellow APs that can only be used for movement. It is possible that you only have say 3 Green and 1 Yellow AP left, which aren't enough for a 4 AP skill.

Note that you can save Green APs for next round at a rate of 6/3/1 saved APs to get 3/2/1 AP next round. Yellow APs are not carried over.
Also note that movement skills (Dash, Charge,...) do use Green APs by default, even though they fundamentally are "movement".

2 Heavy Armour Classes:
* Defender. A sword and board tank.
* Champion. Melee DPS warrior

2 Medium Armour Classes:
* Vanguard. Rogue-like class, if your idea of rogue also lays traps
* Sage. Support/Buff/Melee class

2 Light Armour Classes:
* Marksman: Ranged bow damage. Fire/Poison dots
* Arcanist: Spellcaster / debuffer.

Similar, but different. Sometimes it is getting the same skill at different tier. For example Cleave. Sir Kay has it on tier 1 and Balan on Tier 2.

Sometimes it changes the class flavour quite a bit. Like Mordred has lightning based skills, Galahad Fire skills, Red Knight some DPS skills including a throwing axe, Palleas is more of a classical tank, with taunts and armour skills.... Despite them all being Defenders.

Events often pop up between missions. Some need to be dealt with immediately, the others that appear on the world map you can navigate to and deal with. The latter have a 2 mission timer (meaning they will disappear after you do two missions)

Worth knowing:
* The class/alignment of the hero you send is relevant for success! There is a hint in the text. For example: If the event involves diplomatic dealing with druids, don't send a Christian. If it involves a duel, send a Champion. If it involves scouting send a Vanguard or a marksman. If it involves a ritual send an Arcanist or a sage.
* Rewards are often: 40% Exp (as in 40% of a level), gold, loyalty, items,... if the hero succeeds. Hero still gets Exp, but may lose loyalty and is more likely to get injured if he fails.
* The events with a 2 mission timer. It is advised to do them on the turn they appear or the following turn. To avoid the event being overwritten.

That hero is a Sage. To recruit them:

* Tristan after recruiting him asks you to be allowed to bring Isolde's corpse to the crypt. If you allow him, he'll bring her corpse to the same place where your dead heroes go. For me the events procced very early in act 2, it could be related to his level. Keep him in the training grounds just in case it is level related, if you're not using him.

* Use a resurrect Token to resurrect her. There is a one use item that allows to resurrect any dead hero in the crypt, it can be used on her

* I am unaware of any in-game reward or banter for re-uniting them. But it maybe satisfying if you're the romantic sort :)

Officially 4 Acts with a boss and a cutscene at the end of each. I finished the acts at week 54, around 50 hours played.

BUT you can keep playing after that. There are 20 missions available after "finishing the game" and also 3 boss encounters (the boss encounters are just a fight, no map to explore). Each of these 23 weeks guarantee a Relic.

At week 77 for me there were no more missions... for now. But that was 80 or so hours worth of content.

There is some replayability value going for the opposite alignment: Different heroes to recruit and align with but the heroes from the opposite axis become mini-bosses you'd have to fight off in a series of missions.

Keep a few hundred golds in reserve as you start your mission. In some missions you get to hire a mercenary (another way to get a 5th member), or pay a teacher to buy 1 skillpoint (!), or pay off an opponent to avoid a fight.

Also explore the map even when the main mission of a given map is done: Side quests, side duels, side encounters, the odd vitality healing shrine or treasure chest (sometimes requiring high Spellcraft or Perception) may remain unclaimed if you just hit the "Finish Mission" when it becomes available.