30. fxg7 was a good preliminary check.

Probably the best, but it might have been worth following through on the other options to see if they really were fatal. [A] 31...Ke8 does look distinctly unhealthy: 32.g8=Q+ Kd7 33.Nxb6+ etc. But [B] 31...Kg8 32.Ne7+ Kxg7 33.Nxc6 Bxa5 34.Nxa5 Rc8 and, with the exchange for a pawn, Black is none too badly placed. Not what he might have hoped, with the huge advantage earlier on, but still with a chance to win.

(!)

The amazing 33.Rxa6 would have been worth a look, simultaneously attacking rook, bishop and queen. The situation looks wickedly complicated. For instance, after 33.Rxa6 Qc6 34.Rxb6 Qxa4 35.Rb8 Qxe4+ etc; or 33.Rxa6 Qc6 34.Rxa8 Qxa8 35.Nxb6 Qxe4+ etc. 33...Rxa6 won't do on account of 34.c8=Q.

33. Bxa5!

Agreed, after 33...Bxa5 White's ingenious attack runs out of steam. You have to admire his resourcefulness, though, in the face of enormous odds...

Better yet, 34.Rxa6! should win for White...

Couldn't be helped: the bishop was attacked and pinned anyway, with the a-pawn about to pounce.