I think you guys are over analyzing this one. Keep It Simple!
I have used 4200 on my thru hull fittings for years. When I put the grinder & course wheel on the thru hull external flange, it takes 3 minutes tops to grind it off and then pull the remaining thru hull, valve, & tailpiece from the inside of the hull. The 4200 is heated and releases easily. No busted knuckles and no profanities! Just changed out two a couple weeks ago on a recent haul out. With a good seal on the concave side of the thru hull fitting externally and seal inside the 1/16" gap, and sealer between the interior backing plate and thru hull nut or adaptor plate, you're good to go. With a shear strength of 1900 psi and tensile strength of 4200 psi, that assembly is not going to leak and it certainly will not fail between the external flange and interior backing plate and nut. Just my humble opinion!