Re: How much deck over the cabin do I remove?
On my boat I've been thinking of drilling a regular grid of 1/4 inch holes (maybe from inside the cabin), 1 inch or so apart. I saw this in a West System PDF as a way of drying out the core where it is not yet rotten. Seems like a good idea, but we will see!
Maybe that would work in this case. At the very least, drilling a preliminary grid of holes maybe 4 inches apart would map out the extent of the job.
BTW, how the job is done depends on how much experience one has with fiberglass and epoxy. It can go from very hard to not that bad depending on the skills and approach used.