The answer to number 1 is that I've moved away from that harm model for the final draft. Instead, a more traditional damage track is back in the game. Seven boxes, unstable is marked when you take 4-harm and any harm after 7 will kill the hunter.
(I should have a new draft up early next week with this in it, but that should do for now).
Shorter version, yes they stack.
For number 2, it's a judgement call. I'd ask "how do you learn to do that?" If there's a good answer, I'd allow it. It depends what they are, too. A Mundane taking a supernatural move from the Monstrous book is one thing, but for a Spooky it might be perfectly natural.