Thanks for all the comments, gang. I think I'm going to proceed, but carefully:
1. Order in an XP2 now while the $300 rebate is on with a 23 and 50 and see how I get on with them with a 14 or so day deadline for making a decision so I'm within B&H's return window. (I'm not one to buy and return. I can think of only one photographic item I've returned in 10+ years that wasn't defective.)
2. At the same time start listing my excess gear that I've been meaning to sell anyways. Extra M and LTM lenses, RapidWinder, Motor M, NEX gear, Nikon 1 kit - I have a lot of clutter that isn't getting used.
3. Assuming 1. works out (and I have every reason to think it will), get the M-P 240 listed for sale.
4. Keep the M6, Leicavit, 35/2 ASPH and the 75/2 ASPH for now. Possibly the 21, too. See how I get on with going back to film only on M. If that doesn't work out or doesn't get used, divest the 21 and 75. I'll keep the XP1 too.
I do have an attachment to my M6 and 35/2, but I don't want to keep much more than that if it doesn't get used. I was considering selling the M-P 240 for an M10 to see if that made a difference, but that would be a *very* expensive experiment. I think the XP2 makes more sense.