And third, how to get to Shrine of Balance?

I'm actually giving this great mod another go myself, in lieu of a recording for YouTube (having trouble remembering where one of the gold keys is). As for your dilemma, here are some hints:-

1) Don't three brothers need rescuing first?
2) What would you use to mine gems?
3) How about the four elemental shrines?

By all means let me know if you need additional help.

Three brothers?

Once you solve at least seven quests (open four windgates / recover three figurines), you'll gain access to the best gold key loot. In the same room is another stonehead who gives you a key and tells you to rescue some of their kind from the Pyramid. This is the only way to obtain the Serpent Staff.