Caro Kann IQP and some other questions

@1

"Why is it best to go 6.Nb4 after Bd3?" ++ To threaten to trade off white's strong Bd3 and gain the bishop's pair.

"White can just play Be2 and black knight is misplaced." ++ Black can play ...Bf5 attacking c2

"Engine suggests 9. Bc5 for black, trading the bishops"
++ You mean 9...Bf5, to trade off black's bad bishop Bc8 on the color of its own pawns d5, f7 against white's good bishop Bd3, not on the color of its own pawns d4, f2.

"there's a doubled pawn on the f file and a weak isolated h pawn"
++ A doubled pawn is not that bad: black can play ...e6 and is solid.
The isolated h-pawn is not weak. It is not on an open file and white cannot get at it.

"What's the purpose of this move?" ++ Trade bad bishop Bc8 against good bishop Bd3.

"isn't it a big weakness?" ++ No, not at all.

"In IQP positions, what's the game plan for the side with the iqp" (after 12.e5)?"
++ The general plan for the side with the isolated queen's pawn is to make good use of the active pieces, the open e- and c-files and the outposts e4 and c4.

"24.a3 isn't, why is that?" ++ After 25 b4 your own pawn a3 is weak.

"makes eval go from -1.9 to -1.3" ++ That is just the same: white is losing.

Thanks