Turns out this an older boat than 2015 and it is no longer under warranty. Dealer agrees it needs a new hull, at my expense.
So I decided to try a repair since I have nothing to lose. Here is what I tried, and it survived one good test so far.
Most of the insert is still in the hull. It had a non-brass piece inside it, presumably to prevent water flow (?), so that even though both the inside and the outside are threaded, you can't see through it.
I carefully drilled out that piece using a protective nylon spacer and a 1/8" drill bit to do a pilot hole and then a little larger bit to widen the hole. I then cleaned up the threads with a hand-tapping device for 1/4"-20 threads. This lead to a clean brass insert threaded all the way through.
I then put a 2" SS flat-head machine screw up through the bottom, though mast receptacle and the v-brace and then a lock nut. The flat head is pretty much flush with the hull and fits the hole perfectly. This is probably not the right kind of SS, but it is what I could get on labor day weekend.
On this first pass I did not use any loctite or any sealing gel. I took it out and it works great so far with no leaking.
With the new bolt going all the way through, I think this should actually be stronger. I could also add a plate or something as suggested by the earlier post. (I am using the stock sail.)
My plan is to redo the bolt with something to better seal the hole and also something to fill the gap left by the missing brass part (although I guess I could put the broken half back in as a spacer).
Would love recommendations on how best to seal this bolt and on what kind of SS bolt to use (since it is in the salt water). I assume blue loctite would be good as well.
Plus any feedback on why this was a terrible idea is welcome too
[but the alternative is a new hull, which is still an option]