I guess I used rivet as a generic term when I should have differentiated rivets from retainers. Sorry about that.

You are correct, the Rivets that require a rivet gun to install are not reusable. The Christmas tree ones are often damaged when you pull them but GM likes to use those on wire looms and replacements are hard to source locally.
The other retainers are reusable.

On those fender retainers that bend the fender flange you may try the green handles pliers above. They don't always fit but they press and pull at the same location so they don't distort the fender.