I would strongly suggest getting time in another Comanche if possible. I'm not sure flying a Cutlass or Arrow (or possibly weirder complex plane) around will do you any favors when it comes time to collect your new bird. It's not just practicing GUMP, it's a whole set of muscle memories, flows, and habits being developed that you'll need to redo when you get your plane.
The PlusOne deal sounds.. frankly, perfect.
If they won't sign you up for a month (and you can't cancel after 'wanting to join for a year' after a month without penalties), then I'd try to beg, borrow, or steal another PA24. Failing either of those options, I'd just wait for your new plane to exit Annual and fly with a CFI in Idaho.
$0.02