I think your best solution is these:
Continentals ExtremeContact DWS and a Space Saver Spare tire.
It really sucks that they didn't tell you about the performance tires but I don't think you should pay those crazy prices they are quoting you either or run 19s for half the year. If you just want 20s all year round and not to fuss with changing wheels or tires every season, this is a great solution.