Re: Chainplate advice needed
I'm going through the exact same thing and my chainplates looked roughly similar. That is, a decent amount of visible surface corrosion but perhaps no deep corrosion. Regardless I think I'm getting new ones. I don't see a strong reason not too, it's just machined stainless won't be terribly expensive.
You could consider replacing the hardware too, I love McMaster.com for stainless hardware.
For sealant I'm considering butyl with life caulk over that so on the deck I won't have sticky butyl. Though I'm still unsure.
One thing not to do: seal from the inside. My PO did that and it helped cause significant core damage in the chainplate area - if water gets into the deck you want it to have a chance to get out again, and you want to know it was there. Never seal from the inside.
1979 Contessa 26