Can I glue the shower escutcheon onto a fiberglass shower stall?
Yeah, glue's a bad choice for exactly the reason you noted...
If you can't get the bolts extracted and replaced with a decent brass or stainless steel that won't corrode, I'd suggest having a helper hold the escutcheon in place while you caulk around it. Then, make a bridge of tape (i.e. not touching the caulk) to hold it in place while the caulk sets up. Once the caulk has hit its minimum cure time (preferable to wait 24 hours or until the max cure time listed on the tube), peel the tape off.
When the time comes, you should be able to scrape off all the old caulk with a razor blade to have a nice clean surface to do it all over again.