I rebuilt a 2001 Crestliner 1800 Superhawk, the construction should be similar. The boat has aluminum stringers and when I re-decked, I drilled all new holes going into the stringers. 3/4 plywood is overkill, use 1/2 plywood . I used 3/4, but my deck panels are larger, and I wanted solid decks. They are rock solid, but not sure I would go 3/4" again, extra weight.
I used dougfir marine plywood and treated it with an antifreeze/borate solution, then Termin8. I'm in the desert, and don't have to deal with constant wet conditions, but still wanted the wood protected. OTF is a good option, heavy initial coat of thinned SPAR that will penetrate deep into the plywood. Not sure what plywood you have in the UK, but "5-ply 1/2" exterior dougfir plywood" is very rigid, cheap alternative to "marine" plywood. Your boat was built with CCA treated plywood, which is very hard to find. DON'T use ACQ (green) treated plywood as it will literally eat the boat.
I don't remember how the seat bases were attached, screws only or nuts/bolts, but there were aluminum backing plates underneath the decking, that the seat bases attach to. I used #14 SS screws reattaching the seat bases, re-clocked the aluminum plates so the screw holes were fresh. IF all the seat base screws only hit open plywood and no stringers, I would nylock nut/bolt them onto the decks with aluminum backing plates underneath. I drilled all fresh deck screw holes into the stringers, so everything looked symetrical (vinyl covered decks) and I had all good stringer screw holes. If using carpet, you can re-use the existing screw holes (if not stripped) as the carpet will hide the screw heads, use old deck panels for templates. The deck screws from the factory are haphazardly placed here and there, not a biggie as carpet hides them.
Be careful removing the deck screws, they are a specialized self tapping screw and hard to find. Dig dirt out the screw heads, then removed the screws, I still managed to strip out a bunch. I went back with new #10 truss head screws, instead of the OEM screws. Used a 3" long zinc coated #10 steel screw of the same pitch, with the head chopped off, as a tap for threading/tapping the stringer screw holes.
Fiberglassing the decking is the best option, but expensive. All deck penetrations have to be completely sealed if using fiberglass, or moisture will get in and not be able to get out, then the rot begins. OTF should penetrate deeper than poly resin, but OTF's surface won't be as strong. I cut some of the 3/4 marine plywood I treated (flooded) with Termina8 and it went in 3-4 layers deep. Treat the wood so it can't rot, or seal the wood so water can't get in to rot the wood. Crestliner used the first option, so the wood will get wet, but it can dry out again. Looks like it worked well in your boat, except for 1 deck panel. I remember the same with my boat, only 1 deck panel was showing signs of rot.
If you need to remove any of the clear carpet adhesive Crestliner used, lacquer thinner will cut it, you just have to keep the surface wettted. I put paper towels on the surfaces (boat sides), moistened the towels with lacquer thinner in a spray bottle, then covered with plastic - garbage bag. Don't do this by flames or in enclosed areas as lacquer thinner is extremely flammable. I used a respirator, and couldn't smell the lacquer thinner. Also, tons or nitrile gloves.
That's why I have this picture. I didn't discover the holes in the fuel tank until I completed the rebuild and topped off the tank, and it leaked into the bilge... (now where is the nuclear explosion emoji?)
