Goodle stuff here Kurt!
Your remodels are inspirational, unusual and right-on.
Have you taken a magnet to the plates?
We've talked much about plates and metals in other threads. With stainless you are always taking a chance, I'm convinced of that.
316L or megayacht stainless like aquamet is the only possibility for through deck aps.
They may be 316 if no magnets attract, or 304 if magnets do. Trouble is marine monel can be magnetic and highly resistant to just about everything at the same time.
We do have a history of monel being used on Ariels (front tank). Monel is nickel and copper alloy mostly with a small percent of iron usually.
You do have that expected flash of rust showing right where the plate is cut off from oxygen in the deck, but NO flash where you might expect on the face against the plywood bulkhead.
No deterioration in the balsa! I would guess your plates have finishing plates around them on deck. With good caulk I think those little plates do a lot to keep water from following the chainplates through the deck.
I think that if I had to use any stainless where some part of it had to be deprived of air I would squeeky clean the area with the best degreaser and SCRUB 5200 or better yet LifeSeal onto the surface with scotchbrite pad. Really work it in. Something similar to the bearing surfaces also, making absolutely sure no liquid of any description ever touch that place again on earth or at sea. It's the chloride we don't want soaking the s.s. in close spaces.
Choice of LifeSeal (silicone/urethane hybrid) is based on its expected flexible life, which is the best of all the synthetic caulks.
Snapshots of your interior!
Unless you got a patent on it.
Or some great ideas we might steal,
which I will!