1)There could be more and more variations about game types , for example:
-"Idols lose 1 hp after every 3 turns (both players)"
-"Your idols highest hp may be | 10 -
<number of yours units on the batlefield> " *
or , "You win when you deal 50 hit points (sume of hitpoints dealt to units, walls, idols etc.)"*
In this type, strategy will contains not only "what unit destroy" but also "what units not to destroy"
2)Multi-colored decks give you lot of possibilities , and you don't need to using powerful scrolls. When it will be possible to use Quake for 4 En and 3 Gth that make no sense to using one-colored decks but you have less possibilities , It won't give you any benefit. That's why This isn't a good idea. Many people colect multi-colored decks right now, and some of these decks are very strong.
3) I agree with selebu, he explain everything
4) I will only put a information about how many scrolls is your opponent keeping at the moment, your idea will enforce turning battleground what follows 3D units etc. After all seeing your opponent's backs isn't necessary, too much effects may have a bad influence on gameplay.
5) I think they add this thing soon