wwall: 15...h6? looks like the losing move. Perhaps 15...N4c6 16.Qc2 g6 17.hxg6 hxg6 18.Qd2 Kg7. Or 15...f5, and if 16.h6 (16.exf6 Rxf6 17.Ng5 h6), then g6. If 15...g6?, then 16.Qh6 and 17.Ng5 wins. After 17.Qc2, White threatens 18.Qh7 mate.
If 17...g6, then 18.hxg6 f5 (18...fxg6?? 19.Qxg6+ Kh8 20.Qh7 mate) 19.exf6 Rxf6 20.Qd2 Kg7 21.Ng3, threatening 22.Nh5+ and winning. If 17...f5, then 18.exf6 Rxf6 19.Qh7+ Kf8 20.Qh8+ Ke7 (20...Kf7 21.Bg6+ wins) 21.Qxg7+ Rf7 22.Qxh6 Qxb2 23.Qg5+ Kd7 24.Ra2 Qb3 25.h6, winning.
Perhaps 20...Qb3. If 21.Bg6 (21.Nf4 Ne8 22.Bg6 Nd8 23.Bc2! Qxc2 24.Rac1 wins), then 21...Rf8 22.Ng5 hxg5 23.h6 Qxb2 24.h7 Qxe2 25.h8=Q Rxh8 26.Qxh8 fxg6 27.Qxc8 Qa6 should hold. If 20...Qb5, then 21.Nf4, threatening 22.Ng6+ and winning. If 20...Ke8, then 21.Nf4 Ne7 22.Qxh6 Nbc8 23.Qh8+ Kd7 24.Qf6 Nxd4 25.Nxd4 Qxd4 26.Qxf7 Ra6 27.Ng6 Re8 28.h6 wins.
Slightly better is 21...Ke8. Now 22.Bg6? fxg6 23.Qxg6+ Ke7 draws. If 22.Qh8+, then 22...Kd7 23.Ra2 Qxa2 24.Qxd8+ Kxd8 25.Bxa2 b5, and Black may survive. Best would be 22.Ra2 Qb3 23.Qh8+ Ke7 24.Qxh6 Rg8 25.Qf6+ Ke8 26.Ng5 Nd8 27.h6 Nd7 28.Qg7! and winning.
After 22.Bg5, White threatens 23.Qxf7 mate.
If 23...Qxe2, 24.Nxf7, threatening 25.Nxh6+ Rf7 26.Qxf7 mate.
After 24.h6, Black cannot stop 25.h7 and 26.h8=Q