Welcome to the BCC Owners’ Forum, Ignazio.
Ouch! Very sad to hear of your problems. Congratulations on your purchase of White Wings III.
I’ve never removed that teak plywood that was applied to the interior wall of the coachroof. I hope another owner will be able to shed light on how it was attached and how best you might remove it.
Your problem is not limited to the interior, of course. My reading of your post is that you have deck leaks at one deadlight (in the forepeak) and one or two portlights (in the main cabin).
You cannot stop a deck leak from inside.
My guess is that the caulking (the sealant) has failed. Think of an opening portlight as made of four things: (1) a spigot (a cylinder of bronze) that penetrates through a sandwich of teak plywood and GRP; (2 & 3) two bronze rings (one of which is integral with the spigot) that form the outside layers of a sandwich with the inside layers being the GRP monocoque and that teak plywood; and (4) the hinged light or glass & bronze door.
I’ve not a good illustration. I’ll attach a screen grab from the ABI catalogue (ABI being A & B Industries, who now longer exist as such but who were the manufacturers/wholesalers of your portlights and deadlights).
I think your task is to (a) remove the portlight completely; (b) remove so much of the teak plywood as is necessary; (c) clean up the GRP; (d) replace the plywood with plywood of a similar thickness; (e) re-install the portlight, using copious quantity of sealant; (f) cosmetic work.
Sam L Morse Co, who built White Wings III, routinely added teak plywood as a liner of the coachroof (main cabin) and the coaming around the scuttle hatch. I do not know how the teak plywood (or teak-faced marine plywood, if you prefer) was attached to the GRP monocoque.
White Wings III might have two layers of teak ply in those areas. Zygote, which was built at almost the same time as White Wings III, has a slightly different construction: the GRP deck monocoque, an internal lining of teak ply, and then a thin GRP cosmetic layer. So the internal layers of the sandwich at Zygote’s portlights and deadlights is GRP-teak-GRP. So I suspect that your sandwich might be GRP-plywood-plywood.
Late edit: I’ve also attached a graphic from toplicht.de showing something of the structure of their portlights.
