@sTuKat Since you are leasing, how about continuing using your Uberturbines in the summer, and getting the cheapest 18" wheels you can find that fit the M3P hubs for your winter / all-season tires?
That way you'll get some use out of the original tires at least, without having to deal with selling them and rebuying used ones. Keep it simple for a leased car, don't spend any more $ modifying it than absolutely necessary.
Alternatively, if you really don't want to use the Uberturbines at all, see if someone with worn out 20" tires with low tread (but not down to cords or such) wants to swap their old tires + cash for your fresh 20" tires. Then run your own smaller wheels + tires for the duration of your lease, and put back on your Uberturbines with the worn out tires when lease is up. Honestly I'm not sure this would be worth the hassle to me, but it's up to you!