Qh4+ is more usual.

Wrong, black can hold onto the pawn, but it has been justified that in attempts to do so he would have to damage his position. Bd6 is clearly a terrible move, plonking it right in front of his d pawn cramping his position and now his B & N are vulnerable to a fork.

Nf3 was better. Clearing King Side for castling where your R would have a nice open f file to aim at the weak f7 pawn, threatening the fork and your attack can commence quicker.

8. a4 is OK, better however would have been a3 which stops the b4 advance which he should have played straight away.

Instead of 8. Bxf4 the preliminary exchange axb5 was good, giving your Rook a semi-open file and winning back your pawn in a better way after 8. axb5 cxb5 9. Nxb5 and then his f4 pawn would become weak and ready to be picked off & u have a potential 1 pawn advantage.

I would mind, that's a terrible endgame disadvantage now for u and it puts added pressure for your attack to work.

hard to see what he could have done about it.