what the hell is the thing called that mounts the fender to the frame on the bottom of the car. it looks like a cross between a flat clip with a nut welded to it.

Would you believe it's called a clip nut? Some people refer to it as a cage nut, but that's not entirely correct.

I'm not sure the original 4-sided clip nuts are still available. Haven't seen them in several years. A good replacement is called "extruded U nut."