Also, as far as I know (thought I don't really know) there isn't a stainless plate that these screws go into. I and some others originally thought the screws were directly threaded into the fiberglass but a surveyor who was recently aboard saw what looks like a thin jacket surrounding the screws which he figures is a stainless sleeve into which they are threaded.
Patience I have. Loads of time, I really don't have right now, but I still want to have a crack at it. Time consuming jobs are expensive jobs in the yard unfortunately. You guys should see the yard estimates to undo this mess!
Med, if it's a thin jacket of stainless and you're time constrained I'll give you something to think about.
Why not take a hole saw that's slightly larger than the sleeve and drill both sleeve and bolt out?
On my boat the cockpit sole is cored (I think, never had to go there). If yours is too you can use a bent nail on a drill to take out some of the coring around the hole.
Now you have two options:
1 - If you have room above the headliner:
Seal the bottom of the hole and fill it with epoxy thickened with microballons. The thickened epoxy won't try and run out around whatever dam you're using on the inside (preventing a mess). Let it cure overnight and drill a hole through the cured epoxy just big enough for the bolt. Secure it on the inside with a nut, fender washer and Loctite. Your favorite sealer spluged in the hole will keep it water tight.
2 - If there's not much room with the headliner:
Thread a throwaway bolt into the proper sized nut after coating the threads with wax (lots of wax). Insert it so the nut will be trapped in the core layer and squeeze thickened epoxy around it, trapping the nut. Do the same for all four and let it set up overnight. The wax will allow you to back the sacrificial bolts out. Just make sure they're all at 90 degrees to the deck.
You can keep overflow epoxy from dripping into the berth by duct taping disposable plastic containers
over the hole from below.
You can keep the nuts perpendicular to the deck by using a scrap piece of plywood.
The toughest part of the job is that first cut with the hole saw...