1) +2.11 (24 ply) 17...h6 18.Qh5 Rae8 19.Nh7 f5 20.Qg6 Qd7 21.Nf6+ Rxf6 22.Bxf6 Qf7 23.Qxf7+ Kxf7 24.Be5 Be7 25.Bc7 Bd8 26.Bxd8 Rxd8 27.g4 c4 28.Bc2 g6 29.Kf2 Nc6 30.Rg1 b5 31.gxf5 exf5

2) +2.57 (23 ply) 17...f5 18.dxc5 bxc5 19.Nxe6 Qf7 20.Nxf8 c4 21.Bc2 Rxf8 22.g4 Re8 23.Bxf5 Bc5 24.Bf2 Bd6 25.Bg3 Nb3 26.Bc2 Nc5 27.Bh4 Qe6 28.Rf3 Ne4 29.Bb1 Bc5 30.f5 Qh6 31.Bxe4 Rxe4 32.Bf2

this was a case where I had to run the computer for a long time for it to get this correct.

As GrahamClayton said, the excessive knight moves really hurt, and the last one (18 Nc4 cost another point. But another knight move is (only relatively) best. After NC4:

1) +3.39 (22 ply) 19.Nxe6 fxe6 20.Qg6 Rf5 21.Qxe6+ Qf7 22.Bxf5 Kf8 23.e4 dxe4 24.Rf2 Re8 25.Qxf7+ Kxf7 26.Bd7 Rb8 27.f5 Bc8 28.Bc6 Bb7 29.Bxb7 Rxb7 30.Re2 cxd4 31.cxd4 Rb8 32.Rf1 b5 33.b3 Nxa3 34.Rxe4

Best is

1) +2.10 (22 ply) 18...Nc6 19.Nxe6 fxe6 20.Qg6 Ne7 21.Qh7+ Kf7 22.f5 Nxf5 23.Bxf5 exf5 24.Rxf5+ Ke6 25.Rdf1 Rae8 26.Qg6+ Kd7 27.Rf7+ Rxf7 28.Rxf7+ Be7 29.Qxg7 Qd8 30.Qe5 c4 31.a4 Bc6 32.e4 dxe4 33.Bg3 Rg8 34.Bf4 Ke8 35.Rh7 Bxa4 36.d5 Rf8 37.Bxh6

2) +2.21 (22 ply) 18...Rae8 19.Nh7 f5 20.Qg6 Qd7 21.g4 Rf7 22.Nf6+ Rxf6 23.Bxf6 Qf7 24.Qxf7+ Kxf7 25.Be5 Be7 26.dxc5 Bxc5 27.Kf2 Nc6 28.b4 Nxe5 29.fxe5 Be7 30.gxf5 exf5 31.Bxf5 Bh4+ 32.Ke2 Rxe5 33.Bc2+ Ke7

As of move 15, black is even:

1) -0.02 (20 ply) 15...b5 16.Bh4 Qe8 17.Rde1 Rc8 18.g4 f6 19.Bg3 Nb6 20.Bc2 c4 21.e4 dxe4 22.Bxe4 Nd5 23.Nd2 g5 24.Bf3 Bxf4 25.Bxd5 Bxd5

<Somacarana vs Cochrane, 1856 saw 5.c4>

c5 6.c3 Nc6 7.O-O a6?! 8.Bd2 O-O 9.a3?! b6 10.h3?! Bb7 11.Be1 Ne4 12.Nbd2 Nxd2 13.Qxd2 Na5?!

<Difficult to understand. 13...a5 or 13...c4 were more natural.>

14.Rd1 Nc4 15.Qe2 Na5?

<The first mistake: removing a piece from the battle. Either 15...cxd4 or 15...Qe8 are better. Interestingly, 15...Qc7 is not bad either, as after 16.Ng5 h6 17.Qh5? the e3 pawn could be captured, with a capture on f4 to follow. In this line Black is better, but if White does not do stupid things like the abovementioned 17.Qh5, the position is nearly equal. Let's mention 15...b5, which leads to tense fight when meets 16.g4.>

16.Bh4 Qc7?

<The second mistake, leading to a nice king hunting. Maybe 16...Qe8 was the best, and now 17.Ng6 does not work, because after 17...f5 the e-pawn is defended. 16...Be7 is also considerable.>

17.Ng5

<The immediate 17.Bxh7+?! was also possible with bizarre-looking lines like 17...Kxh7 18.Ng5+ Kh6 19.f5 f6 20.Nxe6 Qf7 21.dxc5 Be5, and there is total chaos on the board. 17.Ng5 is objectively better, of course.>

17...h6

<Weird, but there is no really better move.>

18.Qh5 Nc4?

<It was time to build some defensive facilities. 18...f5 19.Nxe6 Qf7 20.Qxf7+ Rxf7 21.Nd8 Rf8 22.Nxb7 Nxb7 is not especially beautiful for Black, but with extremely precise play the attack might be hampered for a while.>

19.Nxe6!

<Yes! Exactly now!>

19...fxe6 20.Qg6 Rf5 21.Qxe6+ Rf7

<21...Qf7 is equally tragic, as it loses the rook, but Black is not completely lost (yet).>

22.Qg6 Kf8 23.Qh7 Ke8 24.Bg6 Bf8 25.f5 Kd7 26.Bxf7 Nxe3 27.Be6+ Kc6 28.Qg6 Bd6 29.Rfe1 Nxd1 30.Rxd1 c4

<30...Kb5 c4+!! could happen, and now if 31...Ka5, then 32.Be1+, if 31...Ka4, then 32.b3+! with an unstoppable f6, and if 31...dxc4, then 32.a4+ Kxa4 33.Bxc4 with an incoming Ra1 destroys Black's king. 30...Bc8 also leaves Black defenceless, as 31.Bxd5+! Kxd5 32.dxc5+ with devilish Bf2 ideas will hurt.>

31.Bxd5+! Kxd5

<Sacrificing back the material will not succeed: 31...Kb5 32.Bxb7 with an incoming a4 crushes.>

32.Qe6+ Kc6 33.Qxc4+ Kd7

<33...Bc5 does not help: 34.dxc5 bxc5 35.Qe6+ Kb5 36.c4+ Ka4 37.b3+ Kxa3 38.Qe3 decides.>

34.Qe6+ Kc6 35.d5+ Kb5 36.Qe2+ Ka4 37.Qc2+ Kb5 38.a4+

<With an incoming b4 it is obvious that Black is lost.>