...I am loath to cut the outer deck surface, though everyone is saying it's much easier that way.
Not everyone. Above I outlined the advantages I've personally experienced in doing it from the interior, although it does depend heavily on the location of the rot and ease of access from the interior. I've done them both ways (and both ways on the same boat) and unless its hard to get to the inside area, the interior approach has much to offer the "regular guy" working on his boat in the slip, without a travel lift, without an inside shop location and without a full selection of professional-grade tools and professional advice and experience at his disposal.
Even if the bad core extends to the very forepeak or over bulkheads and so on, I'd still approach the bigger, flat and easy-to-access areas from the interior, and then only have to tear up the molded exterior part from the exterior in smaller sections, minimizing the structural and fairing aspects that are so much more difficult in large sections for amateurs.
Getting a good, calculated scarph angle where the repair joins the original laminations is one main key to the structural glass aspect.
Either way, just starting on it is often the most difficult part, especially since you can get advice here. Doing a small "test area" from start to finish, say 6" or 8" square, in an easy-to-access portion of the job, will be good learning experience without big trouble if things go wrong, and will let you know what you are in for, help develop a proceedure list, tool and materials checklist, reveal other issues, and so on.
We had 10" of wind-driven tidal-flood saltwater in our house last November - putting the boat on the back burner, if not off the stove entirely - and I'm just finishing our home repairs and improvements. I'm looking forward to getting back to work on the boat...