Yea I'm leaning toward this option.I am trying to think of a way to save you lots of work. All I can come up with is this :
Make an FR4 backing plate with a hole in it, that will fit over your through-hull. It will need a recess in it for the nut, as that is already stuck on with 5200. Place a second nut over the backing plate, put plenty of 5200 between the plate and the hull, and tighten down. You now have the load spread over a backing plate, without removing the through-hull.
Currently no valves are on the thru hulls. Just the mushroom head is installed. I have backing plates already made for all of them. I think what I'll do is countersink a hole for the thru hull nut, bolt the seacock flange adapter to the backing plate with countersunk bronze bolts epoxied in from the underside, then screw that on to the existing thru hull. I don't have to dog it down, and like the guy above said, 5200 is strong stuff. I doubt there'd be enough force in this to even come close to stressing the thru hull seal. Also the backing plate would be 5200'd in place on top of the thru hull. In addition I would use fiberglass tape and epoxy over the top of the backing plate to make it a permanent part of the hull. Once that is cured, I can just screw down the bronze ball valve snug to the adapter flange.
These seacocks are all over the boat. They are well protected so nothing would be hitting them. But still, I'd like them as strong as possible. The boat is a 1980 Gulfstar 50, so the hull is old school and pretty thick. No core in it. And like I said, the new thru hulls were done well even though they weren't done to the standard of the Compass Marine guy, whom I tend to trust implicitly. They would be fine without the flanged seacocks but I prefer things to be bullet proof. I think at this point, removing the thru hulls would just damage the hull and create more unnecessary work.