The doubt of the famous #226 problem

[peter]

58.Qa4 mates one move faster than 58.Qc4 so this line is clearly not the best.

I found three suboptimal moves with KQkn EGTB hosted at Shredder website:

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After 57.- ..., Nd3:

[d]8/8/1K6/8/7Q/3nk3/8/8 w - - 5 58

White plays 58.- Qc4 (#17 according to EGTB), while 58.- Qa4 is #16.

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After 63.- Qd4:

[d]8/8/3K4/8/3Q1n2/5k2/8/8 b - - 16 63

Black plays 63.- ..., Nh3 (#10 according to EGTB), while 63.- ..., Ne2 is #11.

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After 64.- ..., Nf2:

[d]8/8/8/4K3/3Q4/5k2/5n2/8 w - - 19 65

White plays 65.- Qf4 (#10 according to EGTB), while 65.- Qc3 is #9.

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So, 58th and 64th white moves delay the checkmate one move each while 63th black move shortens the checkmate one move. Once your line reaches 6-man EGTB with 52.- Kxa6, the optimal line should end at 73th move, not at 74th.