correct given trophy
I did not understand the parameters of the puzzle. I thought it was in the first 7 moves. White usually had just one forced move and black seemed to make poor attacking moves. Missing the clear 1)...Nf7# and his 6)...QxNg2 is suicidal.
6)...Bd6+ forces white's king to first rank, and either black's b or c pawn queens pinning white's queen. White's best hope is then 7)NxBa6 and hoping black does something other than Kb7 so he can sneak in 8)Bc6#. Other 6th move possibilities for black are Ng4+, Rxh3+, even QxNb4 negates white's counter play.
From the "final" position. the solution, mate in 3, is clear and has been given.
8)QxBb7+ KxQb7
9)Bc6+ Kb8
10)Na6# all completely forced.