EDIT: Area is zone 4/5 just north of Portland, OR

I made a mistake on the design for a 2' bump out on our kitchen. In the effort to minimize cost and protect my favorite Japanese Maple I really pushed for a cantilevered bump-out instead of a poured footing.

Problem #1: During construction we realized the existing foundation came up farther than we thought (behind the siding) and had to saw cut it down to the subfloor level (i.e. there are no rim joists, so no ventilation ability from existing crawl to bump-out subfloor)

Problem #2: I was blinded by protecting my favorite tree and failed to account for subfloor framing in my measurements of the exterior grades and now the bottom edge of the bump out is pretty much at grade (it's on a hillside, so only the left half of it). I'm regretting this decision, but it's framed and roofed so I'm trying to find a solution to protect it from moisture and rot long term.

Problem #3: I only have 7.5" to get R30 insulation. My engineer followed my direction and minimized the timbers to keep them high off the ground... so 8x framing is all the depth I have (which is actually saving me from being IN the dirt).

Here is a snapshot of the exterior elevation at the base. The green & pink lines are the grades, and the yellow represents the subfloor framing that is too close to the dirt. The tree I want to protect is on the left which inhibits dirt removal to an extent.

I was initially planning on just filling the joist bays with closed cell spray foam which would both waterproof and mitigate the need for venting so close to the ground, but I'm really unsure if I could create bigger issues by encapsulating it so it couldn't breath.

Like this:

I could also build a small wall around the flooring system to end up with something very similar to a crawl space on a mobile home where the concrete wouldn't be supporting anything.

I really appreciate your help and advice on this.

Thanks!