Wakemakers supplied me with Attwood silicone adhesive sealant. I'll see if I can get the 3M-4200.
You dont necessarily need 3m 4200. Its just one of a dozen proper products that can be used. Instead, you need to see what the applications are of the product you already have. Is it for below waterline applications? Whats its setup and cure time? Are you working in its recommended temp range?

My sense from talking to the guy is that you shouldn't think of needing it to be really locked down to the hull the way you would with main supports for your tower, etc. He was saying that it's only about 3 psi of water pressure and locking down too much on the fiberglass could cause structural damage to it. He may have been trying to protect me from myself though if I've already overtightened once. I agree with what you're saying, and the through hull having those rails inside for that purpose seems like what was intended. Doing it right after you put the sealant on and using a tool to brace the through hull, (plus a wrench for the nut), is how I plan to do it....just not too tight.
The 3 PSI of water pressure is not what you need to be concerned with, but the flow rate of the lake water rushing in, if there is a failure. With that said, I highly doubt anyone is going to be able to tighten down a thru-hull's jam nut with enough force to damage the hull. Even with the proper thru-hull holding tool and wrench.

With that said. Unless im misunderstanding your thread, you had an issue with ball valve turning the thru-hull when you were tightening the ball valve to the thru-hull. To properly tighten the self-sealing tapered pipe thread ball valve to the thru-hull, you have to hold the thru-hull with more then the average hand. Hand tight ball valve is not enough. 3 PSI or not.