Gene:

Will the PVC plastic and the sealant adhere? I think the PVC/Silicone joint is suspect to me.

Also, the PVC isn't nearly as stiff and is more subject to deflection than the plywood. Will you have "ribs" in the PVC to stiffen it and keep if from deflecting?

A slight deflection will put a lot of strain on the perimeter seals because it will tension the deck.

If the only problem with the current situation is deterioration of the plywood, then:

I would consider just epoxy sealing a marine grade piece of plywood on the top and all sides. I would pre drill the holes, and fill them with epoxy too, then fill the screw holes and drill down through the hardened epoxy waterproofed holes.

As for the ribs, that only does good if you provide a way for the whole system to ventilate. Having a honeycomb substrate will help to the extent that there is some positive way for condensed moisture to dissipate. It's worse in colder climates; but you can still grow mold in warm climates with no difficulty.

Natural ventilation is good. It's really a function of the "dew point" in the assembly. I believe you want the dew point in the middle of the assembly where no moisture has reached.