Since no one else seems to be responding, I will give it a shot. Question #1 – If the subflooring was installed after the walls were framed, you can just pry the boards up after removing the baseboard and shoe mold, if any. If the walls were framed after the subfloor, then you will have to cut each board flush with the wall sill plate.
Question #2 – Treated plywood is OK to use, but not necessary.
Question #3 – Yes