ok, so there was a fiberglass seat adhered to the bottom of the boat,,,i am surprised that an adhesive would tear up some of the fiberglass in the hull,,unless the seat is attached with the same ester resin that the hull is made out of. Either way if this seat is attached so well that it can delaminate the hull then I would think grinding out the remainder( oh boy) would be the way to go. If acetone can soften the adhesive you'll have to get the acetone on the adhesive,not just at the edge of the joint between seat and hull. A 60 grit sanding pad on a random orbital sander might work,or a rough 2" sanding wheel. Wear a filter mask and take a shower afterwards,itchy stuff everywhere.