Instead of 12mm (7/16ths) OSB use 23/32" t&g OSB ~19mm.
'DO NOT' place hardwood flooring over the framing. You need 3/4" subfloor for support and as a substrate for flooring fasteners.
There is a paper barrier (like roofing underlayment) rolled out on top of the subfloor prior to installing the finish flooring.
I can't tell you how to install the floor framing vapor barrier and insulation. That's out of my area of expertise. I'm not even gonna guess.
Keep in mind it will be >impossible< to attach anything to the bottom of the floor framing since it is very close to the ground. Don't even begin to think about rolling something out under the framing, holding it up in place, and gun nailing it to the bottom of the joists. Just trust me on that.
I painted the bottom of my 2x6 flooring to protect it from ground
. My platform is only 8" off the ground and no rot issue. I live in a semi arid climate. No insulation either. For a permanent residence you DO need to insulate the floor. Research it.
Yes rodents can nest in fiberglass. Beats me if they will do so under a
.
Good luck.