Okay, a few things here... First off, you're in the Corner Carving sub-forum, so just keep in mind that most recommendations are going to be coming from a handling-performance standpoint, where tire wear is a vanishingly distant concern...
That said, the big thing you're forgetting about is sidewall compliance. The stock P-Zero tires have (relatively) tall, soft sidwalls, that will let the tire deform a decent amount, and put the entire tread face on the ground, even with a touch of negative camber. By going to 20" rims, with very short, stiff sidewalls, you've removed most of that compliance, and as a result, the inner edge takes more of a beating. This is kind of the nature of the beast with that kind of setup.
Other observations: 1) If you have steel belts showing, your tires are DONE, and need to be replaced, and I mean yesterday! 2) If your alignment tech told you that staggered setups can't be aligned, you need to RUN, SCREAMING from that shop, and find one that has a clue. 3) With the short, stiff sidewall, you will need to run LESS camber and toe to get the bulk of the tire on the ground. Think 1/2* of camber, and essentially no toe, or a 1/16" total toe-in as targets. 4) Toe is the only adjustable angle on the car as it comes from Ford. For your situation ("I like my corners") I would probably suggest installing crash-bolts in the front struts to take up the excess camber without resorting to race-type parts (C-C plates) that will piss you off with NVH.
The inner front wear doesn't surprise me, but the fronts being bald when the rears aren't kind of does. Sounds like you may have excess toe dialed in, and are scrubbing them across the road. If you run your hand across what's left of the tread, from inside to outside and back again (being careful of the steel belts popping out), do you feel one side of the tread blocks having a sharp edge, and the other smooth? If so, toe is your culprit.
