I'm considering going the same route as you, Hank (squaring off the P85+ with 245-width rear tires) due to the wider selection of tires available.
Did you ever get answers to your questions above? If so, what did you discover?
Let's see, that was several months ago.
1. In the "Michelin Pilot Sport PS2 vs Pilot Super Sport" thread, someone posted that I can mount the 245/35/21s on the rear 21x9" Turbine rims. I just want to confirm that's not a problem
Yes, you can mount the 245s on the 9" rims, but it was undetermined what would happen if they were rotated to the front, which has 8.5" rims. So you can go for all four 245s, but still can't rotate front-to-back unless you're willing to risk an unsupported configuration.
2. Given #1, are there any serious downsides/tradeoffs, besides just having slightly narrower rear tires with possibly less grip?
People have told me that the grip is essentially the same, rather, it's cornering that changes with narrower tires. But I don't slam on my MS or race it or track it, so the potential slight reduction in cornering/handling hasn't been noticed by me.
3. Does the toe-in settings for the P85+ need to be adjusted if I don't have the stock 265s on the rears? If so, how do I find out the correct settings for alignment?
I've been told that there are no adjustment necessary.
4. What happens if I rotate the rims front-to-back, but they all have the same size tire? Is that kosher? (i.e. 8.5" rims on the rear and 9" rims on the front)
See #1.
To solve that problem, I traded another MS owner my two 9" rims for his two 8.5" rims, so now I have a matching set all around, and I can rotate front to back.
Oh, and I think I posted this before. If I had to do it all over again, I would seriously consider keeping the 9" rims in the rear and putting on the Hankook 275s. That then makes them non-rotatable, but maintains the staggered set up with much less expensive tires. If I ever come across a cheap pair of 9" rims, I may do that. But maybe not, the 245s ride real nice.