Getting a ceased screw out of fiberglass??

Agree that penetrants should not hurt fiberglass.

2 suggestions for removing the screws:

1. Enlarge/re-establish the screwhead with a Dremel tool and a cut-off wheel. If you do this on a crosshead screw, you'll need to lop the point off of a "Phillips" screwdriver to back it off. I bought a cheapie #0 and #2; and performed this tip-otomy. They have worked well.
2. Find LH twist drill bits that are a little bit smaller than the screws. (Need to find an old school hardware store as the big box places don't have 'em.) Fit to a reversible, high torque drill. When the drill bit gets a good bite on the screw, the screw will come free. The screw may also climb up the drill bit. That's ok.

Good luck.