One surmises that Black intended to recapture by ...Nxb6 were White to exchange queens. I would have preferred ...Nc6 myself, as 12.Qxb6 axb6 looks fine for Black, and the queen's knight is more actively placed..

I agree, Nc6 seems more natural to me.

Solid.

That shows a fine judgement! Many players would play 14...Bh7 as a reflex action, and then wonder what went wrong after 14...Bh7?! 15.Nf5 Bf8...

Yes, black is a good player.

A fine post for the knight. White's game looks pretty reasonable hereabouts.

I agree: 18.cxd4 exd4 19.Nxd4?? simply lost. But 19.b4 looks pretty OK to me. Closing down the Q-side as White has done looks counter-intuitive, unless White has ambitions on the K-side. His minor pieces do seem to be pointing that way, so that might be understandable.

20.Nh4 doesn't look too stupid an idea...

Now, leaving the a8-rook unprotected might not have been Black's best option. If he wanted to double rooks on the a-file, perhaps 20...Ra6 was the way to go. White could now get a goodish sort of game by 21.b4! cxb4 22.Bxb4! (this time it works!) ...b6 23.Bd2 with 24.b4 to follow.