Why not put temporary supports under your deck, cut the posts off, then build your walls out of whatever you want? A wall that tall will need to be reinforced with rebar and ladder ties, and the blocks will need to be core filled.
I never considered using permanent wood foundations in this situation. This situation is more of a retaining wall than a foundation. I believe wood will be a nightmare of moisture and rot problems. Bowing and warping of the wood will just make this worse. Where will the water that drips between the deck boards go? Will this be open across one end?
A lot of questions back to you..
I'm afraid that mold and rot, rust and general creepiness will make this space unusable if you proceed as planned.
Sorry about the negativity, but this plan needs more consideration in my opinion.
Welcome to the forum,
I hope we can help you get this thing figured out