Re: new bathroom subflooring for an unusual build
There is no need to use pressure treated anything in this location, as PT materials are only necessary when in contact with concrete slabs and foundations.
There is no need to pull out the original T&G subfloor unless there is rot damage, simply remove the vinyl tile if it is loose, if it is sound, go over the top of it with your new flooring. If there is damage to the original subfloor, you only need replace the portion that is damaged. I would use plywood, since it's the easiest to use for covering larger areas. At the transition between the old and the repair, place a 2x4 or 2x6 joist to support the now raw edges.
Yes, you can fur out the wet wall with standard lumber, not PT, to be more accommodating to your needs, the only caution here is that you do not diminish the bathroom space, meaning a standard tub is 30"x60", don't encroach on that space or your tub isn't going to fit. Premade shower pans will have the same problem. If you do custom tile work, then the interior space doesn't matter because the custom work can accommodate whatever you have.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!