Welcome! Sorry to hear you're having troubles. Let's see what we can do to help, or at least confuse the situation
#1 sounds like a new key setup might be the best option depending on the damage. You might be able to find a whole key set on eBay and just switch everything over.
#2 There's a pretty popular aftermarket dash for the 919, a number of people around here have converted to a digital dash setup. Personally, I like the stock setup and if someone has upgraded, you might find one cheap.
#3 You'll have to give some more details on this. It sounds like a "false neutral" and I've had that happen a few times. Basically you shift into a gear and it doesn't actually shift fully into the gear so it's a neutral but you're not near neutral, you're in 3rd/4th or higher.
Slipping out of 1 or 2 back into neutral is a different thing.
One thing I'd be concerned about the false neutral is letting it go too long because I think it's the gears not meshing. I'd do a fresh oil change and do a firm shift. I think most agree that the Honda transmission needs a firm shift. The oil is something I noticed, it's harder to shift when the oil is older.
#4 the choke on the 919 does nothing more than open the throttle a bit. I understand that it doesn't make the air fuel any richer, so it's not really doing anything but kicking up the revs, which can help warm things up quicker. I was under the impression that the EFI took care of the temp concerns.