The retaining nut you refer to are indeed "riv nuts" (actually rivet nuts) and they basically work like a pop rivet. The tightening of the bolt expands the rivet nut in the hole to hold it tight. They are used in a blind hole (no access to the back side of the structural material). The easiest way to remove a stuck bolt from a riv nut is to apply continuous upward pressure between the attached item and the base it's attached to. A cold chisel or screwdriver usually works. Then spin the bolt off with a impact wrench. One of two things usually happens. Either the bolt come out or you pull the riv nut completely from the hole. It really doesn't matter because the parts off. The trick is how fast the impact turns the bolt.