"The porch is 10 ft up with plenty of ventilation underneath. In the conversion, we removed the original (6 x 1-1/4 in round-nose) decking & laid 3/4 in OSB T&G plywood. Then erected open stud walls, built a roof
(lean-to off the house roof), and screened all around.
Now the floor is delaminating & rotting, and the base plates for the studs are rotting as well. This is due to collection of rainwater on the floor during storms
RicodJour's NexWood suggestion is intriguing, but there don't seem to be any NexWood dealers in my neighborhood (Georgia). I have seen similar produts locally, though - this is worth a look-see.
Still open to other suggestions, though. Is there any way I can put down a concrete subfloor over wooden framing? "
I'd suggest instead of throwing good money after bad, you get some estimates from professionals for straightening this out. Particularly since you're now thinking about putting a concrete floor 10' up in the air, supported by God knows what. This is starting to sound like one of those DIY decks that collapses every so often with 10 people on it, sending them into the street. I'll bet you didn't even have a building permit or inspection when you did the original conversion that lasted all of 7 years, did you?