I replaced the starboard chainplate doubler with 3/4" cabinet-grade ply, glassed over it. Although I'm finding a good bit of my transom to be rotted, the area around the chainplate was OK when I did the repair last Spring.
It's been about 3 weeks since I cut several openings in the inner hull in the transom and forward to the front end of the quarterberth. Using a small heater, the core seems to be drying out quite well. The balsa was absolutely saturated when I first opened up.
This past weekend, I drilled into some blisters located under the cooler - the balsa is yellow but wet, as you described.
Still trying to find a warehouse to move into for more thorough drying.