I'd just leave the nuts in, if I were you. Rivets are cool, but if it's rideable and the nuts don't chafe, I'd call it done.
I pounded on the rivet post side, with the top of the rivet on a piece of wood on a heavy bench. Other rivets, I've tuned up by putting a hammer up inside the cantle, and then tapping on the rivet head.
Good luck!
Philip