OKAY I think I've got it working though I'm not sure exactly which step made the difference. I ended up doing an entirely new download and install of Arcade Spirits 1 via the itch app, loading up one of my old saves that was near the ending (because I DID transfer all the old saves into the new RenPy directory on the new C drive) and re-exporting at the end of the credits when prompted. THIS time, when I loaded up New Challengers again and started a new run, Iris asked me if I wanted to import. So hopefully I'm good to go! YAY!