I don't see how it's Pirelli's problem that the local tire places don't carry a particular type/size of tire in stock.
Like every business in the world that has to keep inventory on hand, tire shops only carry the size and type of tires that they sell frequently. PZero summer tires are not common in the real world and neither are wider, low profile tires. And frankly, 90% of people who buy tires want the cheapest option that will last the longest. The reality of owning any car with quality high performance tires is that you will always have to order replacements---be it a challenger SRT, camaro 1LE, mustang GT+track pack, Nissan GTR, 911, etc.
As far as the spare is concerned, I can sympathize as I thought it was also a bit of a cop out from Ford. They should have at least made a real spare an option for ALL mustangs, regardless of options. That way they can send magazines versions of mustangs that will crack off the fastest times and the rest of us can actually use these cars on a regular basis without worry about a sudden flat. One option to consider is using an 18in wheel at the basis for a spare. I have a '12 GT+BBK and run an 18in winter setup, so it is possible.
If you REALLY want to use the OEM Ford spare, I would think that you can still use one on a BBK equipped car, even if one of you fronts went flat. You would just need to move the rear tire on that side to the front and put the donut on the rear tire. Though you would have to make sure that you can lift both the front and rear of the car at the same time, so you might need to carry a jackstand or second jack.
For the record, I haven't tried any of these potential solutions.