Rob's right, 4 bolts to remove the bags.
You'll want to take them off the frames before reinstalling. Trying to put them back on in one piece will be difficult. The frames moving like you described is normal. I always take then completely apart and clean them good, they take a lot of abuse under there.
Under the bags is a stamped metal plate. I always re-paint those every time I can. Their thin metal, always seem to have some light rust and are big $$$. There's some small screws that strip easily and some thin rubber spacers that fall off. Sometimes I put the spacers back, sometimes I don't, doesn't seem to make much difference.
http://www.partzilla.com/parts/search/Honda/Motorcycle/1998/GL1500C+A/REAR+FENDER+%2B+GRAB+RAIL/parts.htmlGo there to get the OEM bolts that go on a Standard for when you want to go bagless, although I'm not sure why you would.
Or you could go to a Ace Hardware that carries Chrome Metric. Make sure the thread pitch is right.
Don't feel embarrassed to ask for help, we've all needed it.