FWIW - If the mfg was no help, I'd try heat and a blade. For any adhesive residue, goof-off type stuff would be my last choice after sandpaper and a router failed. Rubber Cement Pickups or your thumb, w/ nail if needed and elbow grease would be first. Then cooking oil.

As far as the aggressive solvents go I don't know what might react with epoxy or gelcoats but I'd start with simple solvents like alcohol, naphtha, spirits ... before I got to the end of the alphabet type solvents contained in most one product eats all blends. All would be used sparingly.

I don't know what contact cement solvent contains but I know whatever is in it is  gelcoat safe for a brief amount of time and I've used denatured to clean epoxy before resurfacing with no ill effects.

Start small. Do no harm. Let us know.