The bow plate and the two stern plates took the brunt of the degradation. They were spongy on the face, but still structurally sound. Also still wet, even though the boat has been stored in a dry barn for six months.

I experienced the same in my SOF boats.
While all the stringers and frames were dry and fine, the bow and stern plates were rotting away - most likely because once the water gets at the ends of the boat, it hardly ever leaves, thus slowly but surely eating at the plywood.

My solution was to use HDPV plastic for those plates. Since the skin is plastic too (nylon), this eliminates the problem.
As a bonus, the HDPV is much stronger and impact resistant than even the best plywood coated with epoxy. In fact, I suspect my plywood bowplate rot was greatly accelerated by the micro-cracks it got from bumping into things.