Appreciate the comments and suggestions made.
This is proving to be a difficult project for us. We are out in the middle of nowhere. Most commercial roofers don't want to drive the hour necessary to get to us, even though I have acknowledged my willingness to pay a premium for travel. After calling many roofers, I currently have two written estimates in hand with a verbal third.
Every one recommended isoboard insulation of one type or another over the existing roof, leaks and soft areas included, with a single membrane capping everything. One recommended TPO, one PVC, and the third an as yet unspecified granulated top membrane.
The second roofer also offered another option, Deckshield over plywood with sanded seams over the existing roof. Unfortunately, given the fact that the two bedrooms are cantilevered as is the deck above and considering the fact that everything here was done thirty years ago using that period's technology, I'd want the old roof removed. I don't want the deck bearing too much weight. The roofer quoted a price of over $2,700 for removal of under 700 sq. ft. of modified bitumen and its plywood substrate. That would bring this option in at over $15,000 for our small roof deck. Even considering travel, that figure packs a real wallop.
I remain concerned that isoboard over the current roof would deform, especially over soft areas, when used as a deck. Densdeck sounds like a more promising alternative; but, I believe it weighs considerably more than isoboard. I read somewhere that it's about sixty pounds an 8'x4' piece. Again, the old roof would need removal.
If an experienced commercial roofer here has an ingenious way out of this without the need to spend a king's ransom on such a small roof deck, I'd appreciate his sharing it with me.