Why settle for ordinary pressure treated plywood when you can use PWF plywood for the entrance? PWF plywood is plywood meant to be used when building "Permanent Wood Foundations", or houses that have wood walls for their basements instead of concrete walls. PWF plywood is like pressure treated plywood on steroids, being much more heavily pressure treated than your run of the mill pressure treated plywood. You can also buy end cut preservative for PWF plywood too, which has more of the active ingredient in it (copper naphthenate, I expect) than regular end cut preservative.

Canadian Plywood Association - PWF - Permanent Wood Foundations Using Plywood

If you're going to be removing rotted particle board under the exterior walls, then I'd recommend you break the length of the job into 4 foot long sections, and do every 2nd section at a time. That way, the sections that are still in place will support that exterior wall while you're removing any rotted particle board from under it. You can still do all the cutting at once, but I'd be concerned that if you removed all the particle board subflooring from under an exterior wall, that wall will sag and you may end up with cracked drywall joints, broken windows, a door that won't open or close and all the other things that can result from building movement. By doing the job in pieces, you can avoid the wall sagging on you.