Hey, thanks everyone for all the help! Sorry for not being more on top of this; I'm used to forums pinging my e-mail when someone responds to your thread.
Ultimately Ace (my usual hardware go-to) pretends #12 doesn't exist, but HD had very, very limited stock in stainless (nothing in plated). I didn't even consider that there are different grades of stainless.
The old screws actually came right out with a demo driver. This boat has sailed in salt water all its life, so I guess nobody has ever let any water sit inside, at least.
I still can't post images, but if you can forgive kind of sending it on the caulk, you can see the fitment of the replacement sleeve:
https://imgur.com/EVcBuV0I actually do need a full set of gudgeons for a WIP 16, so that's good info. Been having no luck trying to track down a boom anywhere nearby all summer, though.
While I'm here, anyone have an ultimate strategy for solo-stepping the mast on a 14 Turbo? I've been wrapping the halyard around my roof-rack, but it would be convenient not to have to move the truck back in front of the boat.