Are you sure its the frame that is out of alignment, and not the door?
Seems like if the frame is solid against the fiberglass in every area, you're not really going to be able to do much to straighten it. Perhaps over time the fiberglass might warp/buckle a bit I suppose, but that would be up/down along the surface - not left/right pulling/pushing the frame sideways. What I mean is, if there is warping so the fiberglass isn't flat everywhere, then if you pull the frame off, do something to it, and screw it back in place, it's just going to be pulled out of true again in order to go flat against the fiberglass.
Unless some of the screw holes were not round anymore when you pulled the frame off and put it back? If the screws went back in as expected, then I doubt anything can be done with the frame to make things better (other than buying a new frame, and door).
I can't recall - has anyone on this forum ever fixed or reshaped a door frame, where the screw holes no longer lined up??
Seems like over the years I and others have had the doors go slightly off and had to rebuild them. Or in my case, I bought a new door and frame because my original door was having enough things go slightly wrong that it was just way too much trouble to try fixing it. Note my original door was a hollow core door with some kind of foam inside a metal framework, while others here have said they had solid metal/wood doors, so it seems to vary.
Anyway, that's my two cents
Oh, and when I bought my new door, the screw holes did not line up, and in fact the frame was shaped slightly differently - not rounded the same at the top. I did fill in all the screw holes with epoxy, and also had to build out the top sides with some small pieces of wood surrounded with fiberglass filler. More work than I expected, but came out fine and the door clucks shut with a satisfying sound and feel. It was worth it to me to have an entirely new door.