[Illuminated One]

Well, these are not pro tips but...

1) Mostly castles are not worth it as money generators. I don't know the formula but 50% admin is not 50% income but something lowish. Pays of somewhen, but if you can put their extra costs into additional expanders/researchers this pays of better.

3) I don't think this can be easily answered. You should mostly try to research whatever is best in your situation. So for example if you are Marverni at evo-4 you can compare ench-4 with evo-5. Echantment gives you antimagic, poison ward and regeneration. You don't have thugs so don't need regeneration and if you are not at war with someone who uses poison or mr-spells you don't need the others. So why wait on Gifts from Heaven ten turns for spells you have no need for? Of course what you need and can do is largely dependent on the game.
People often rush down construction because all the level 8 items you can make are unique and some a really powerful.