I don't know, but the factory probably had good reason to do different width tires on front and back, as well as different offsets for the wheels. My though is not to worry about rotation as the wear on my tires appear to be pretty even so far. Additionally, should the back wear faster, due to fixed axle, I'll just buy a fresh set for the back.
As for a spare, I would buy a front matching wheel and tire. Should I get that flat tire I would just use the spare on an emergency basis to get home, not worrying about offset or width.