Re: Chain plate or impact.
I honestly don't have an answer at this point Jim.
I know (and knew when I bought it) that at some point there was a leak in the area of one plate.
Like you I know all boats leak, somewhere, sometime.
I can understand water leaking around a chain plate's cover and corroding the top couple of inches of my cap plate (teak) and bulwark (4-6 inches high in that area - and that's before it gets to the 'hull' or the deck level.
How is would continue down the encapsulated - which I take to mean solid fiberglass and resin wrapped plate and then delaminate of cause movement enough to crack gell coat - I'm thinking that's rocket science and only a grinder can tell.
Unfortunately, but the time I have it ground out to see if there's a problem, well it will need to be replaced won't it.
Lessons learned are opportunities earned.