1. Yes, some people do this exclusively. You'll need to be sure you're getting a good seal from your poppets, though. I don't do this because the poppets on my kegs need to be switched out. They don't hold pressure when the lines aren't attached. If you prime in the keg, just bear in mind that you'll get some additional solids in the bottom of the keg.
2. Yes, you could do this. But I think that the folks who condition in the keg leave a bit of pressure in there to keep the lid sealed. That makes it more difficult to calculate volumes of CO2, though. That's another reason I just force carbonate.