I don't understand the flat spot problem. I've stored a variety of cars over the years, from a few weeks to over a year. Never experienced flat spots. I thought this was a myth, or at the least didn't occur on modern tires.
In regards to rust, we've had the 560 four winters. It's a Texas car and is still corrosion free with exception of the muffler. During the winter, it's washed a few times/month.
As the guys say on the M-100 board say, "Garages Kill".