I agree that a5 doesn't make any sense. Your h4 leabes the g4 square open so he could go after that. Or, if he wants to play more defensively c6 followed by d5 at that point in the game should have been fine. Even kicking the knight with h6. When he traded off his defender for your knight it was over.
A very Risky Attacking Game
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I think black is lost after 10 Qh5 ....