Very interesting game.  It looks like on move 44, H4 was the only winning move. And it looks like I7 was the winning move for white on any move thereafter.

link: https://littlegolem.net/jsp/game/game.jsp?gid=1880455

http://www.trmph.com/hex/board#13,c12j11d5d4c5c3e4f2b4b3c4e2g3i2h2f4f3d10k10j10k9j9k11k8c9b11g8f10e10e11d11g7l6i12f8e7e8d8d9c8f7g5g4f6i5

easy win for black. it's hard to understand that JKB played almost all game very vell and he didn't see the final solution

If you are intrested, my game vs Arek: http://www.trmph.com/hex/board#13,m1m1j9e4h6j11i11i4g5g12d10h3i3h4f4h2c4c5d5d4g2e9f9h7d9e8d8e7e5e10l5f3g3f11l8f10l2l6

It's example how the order of moves can change the game. Although 32.f11 was good move, i must play f3 first

I've not checked but this seems not trivial to me http://www.trmph.com/hex/board#13,c12j11d5d4c5c3e4f2b4b3c4e2g3i2h2f4f3d10k10j10k9j9k11k8c9b11g8f10e10e11d11g7l6i12f8e7e8d8d9c8f7g5g4f6i5k3

JKB plays strong openings, but unfortunately later he often seems hesitant about the general plan. Anyway bravo for both players for this good game

Honestly speaking, I estimated postion at move around 40 as not good for B. I thought about i5 for awhile, but didn't see it's working.

Probably I missed this response:

http://www.trmph.com/hex/board#13,c12j11d5d4c5c3e4f2b4b3c4e2g3i2h2f4f3d10k10j10k9j9k11k8c9b11g8f10e10e11d11g7l6i12f8e7e8d8d9c8f7g5g4f6i5h4k3

And I still don't see good response for Arek's proposed variant. The best I can calculate for B is:

http://www.trmph.com/hex/board#13,c12j11d5d4c5c3e4f2b4b3c4e2g3i2h2f4f3d10k10j10k9j9k11k8c9b11g8f10e10e11d11g7l6i12f8e7e8d8d9c8f7g5g4f6i5k3j4h5h6i3j3k1l2k2

I think white H4 is black win this way http://www.trmph.com/hex/board#13,c12j11d5d4c5c3e4f2b4b3c4e2g3i2h2f4f3d10k10j10k9j9k11k8c9b11g8f10e10e11d11g7l6i12f8e7e8d8d9c8f7g5g4f6i5h4i3

But I can't find refutation of my line so maybe it was white win after all.

Yes, there is no response to k3

Maybe 29.e10 was mistake? 29. h9

29.h9

Another option 29.h9