@1
"if white plays D4, can I always force the grunfeld or KID?"
++ You can more often force the King's Indian Defense than the Grünfeld Indian Defense.
For example 1 d4 Nf6 2 c4 g6 3 f3 Bg7 4 e4 is a way for white to avoid the Grünfeld Indian Defense and force black to play the Sämisch Variation of the King's Indian Defense.
"Can ANY opening be forced?"
++ No, but the King's Indian Defense comes closest to that. It also works against 1 c4 and 1 Nf3 and you can also play it against 1 d4 Nf6 2 Bg5 or 2 Bf4 or against 1 b4, 1 f4, 1 b3 etc.
"How much theory do you need to actually meet d4 in reality...?" ++ You do not need any theory, just play and analyse your losses. That which has happened in one of your games is likely to come up in other games, and remembering that lost game will help you remember.
I'm perplexed with opening some theory ideas. Against E4 I always play the Scandinavian since I win lots with it and I know the theory, it's a very forced opening against whites E4 move, more or less. But as black, if white plays D4, can I always force the grunfeld or KID? Can ANY opening be forced? How much theory do you need to actually meet d4 in reality...?