I think that na6-nb4 then a6 is better.

this is a premature move weakening the c pawn of the black , but more importantly opens path for white's queenside bishop to develop quickly, i.e. be3-bd4, to enable e5 and force black to exchange the critical fiancetto bishop. Re8 is the best move here to prevent white's e5.

15.e5 Nd3 16.Nxc4 Nxb2 17.Nxb2 Qxc3 18.Bd4 white achives its goal at center, but black still has counterplay. After 15.Nxc4 Bxc3 16.bxc3 Nxe4 17. Qd4 Nf6, black has a knight in e4 which i didnt like. So decided to materalize the gain by taking with bishop assuming it is a safer option leading to a winning endgame after exchanges, but missed nf6

This was the line I hoped for, I totally missed 15...Nf6 16.Bd4 Ncxe4 17.Ndxe4 Nxe4 18.Bxg7 Kxg7 19.Qd4+ Nf6 after which black has a sustainable game.

Black has to exchange; for example if Qc7 is played, white playes f5, threatening to mate

black position is cramped. b pawn is weak. rest of the game is easy.