So maybe this is a bug as well, but my character is incapable of doing the quick stab finisher and instead only does the drawn out flashy executions, that's fine and all but you can't interrupt them to block. No matter how quickly I tap the button I get the fancy dancey stuff. Anyone else encounter this?

I get the finisher animations, they split between quick and flashy, but I can always interrupt for a parry. He just cuts the animation and moves to the perfect parry animation. It looks a bit weird, but haven't been prevented by priority yet. More the fact he'll sometimes drop out of combat, or sometimes do the finisher animation to an invisible ghost ...