Barrier around shed base to hold dirt for retaining wall?
You want air flow under the shed to keep it dry. The traditional barrier around houses on piers was a wood lattice, mostly aesthetic, and of course unwanted critters would get past it. 

I don't understand your idea of a retaining wall and a barrier with a 2 ft gap. The purpose of this is not apparent to me; it sounds elaborate and a lot of trouble. My initial reaction is that Tyvek wrapped around wood in contact with soil would not protect the wood from rot.

Plants around a shed are going to lead to watering around the shed. I can see that just a lattice around the perimeter might seem stark, but there is a reason this was traditional. Whatever you do, leave some convenient access to put rat traps under the shed. If and when they are needed, put them in the special boxes to prevent pets or any non-target animals from being trapped.