I'm not going to try to figure out what the bits you're describing are, without pictures. I'd just be guessing, and wasting both your time and mine.

Regarding fiberglass: it can be done. I *think* it can be done well, but that's not what happened in the cases I saw. Since I haven't done the work myself, I don't know if the problem was bad materials, bad workmanship . . . or if it's just a case of a bad method of repair.

You have to remember that water WILL get between the new FG and the old plaster, and that tends to cause problems. If you succeed in bonding the FG to the old plaster, you *still* won't have a sound surface, since you have plaster delamination.

Pretty much *any* method of repair that doesn't involve getting down to sound and tight concrete or plaster is a PATCH, not a repair. I would not be surprised at all that there is someone -- either DIYer or tradesman -- who has done successful repairs with FG, but I haven't seen an account of it, or a 'how-to'.

One other point to consider: I've never seen a field applied gel-coat look anything like factory gel-coat. This is cosmetic, rather than structural, so it's up to you whether that's an issue or not. Again, my guess is that someone with the right methods and persistence could improve that, but it's not what I've seen happen.