Nothing you do to the boot is going to help, duct tape=shredded, not near as tough as the leather/pleather it just ate through. Sealer is going to do even less.
See if you can find a pliable 2 part epoxy & put a dab on the rivet, then find level. you might have to prop weird shit under it or tie a shoelace around it & hang one end, so that it settles into a nice round flat circle.
If the rivet spins and the other piece doesn't or vice versa, cut a tiny piece of thin plastic slightly bigger than the rivet Then epoxy it. That should keep it spinning freely.