Is the fight against Genishiro winnable?
The fight at the beginning of the game where, when you lose it, you lose your arm... is it possible to win it?
And if you do, how does the game react to it?
Yes, but the only thing that changes is the cutscene after the fight. You get hit by a sneak attack by an unseen foe, and then Genishiro cuts your arm off.
You can see it in action here: