Both Serhiy and Ned have expressed concern about the difficulty of merging patches from 3.5 to 3.6 once the 3.6 files are renamed. To me, this is a non-issue since I don't expect to do that more than a few times, and some may be null merges of backports of fixes originally developed for and applied to 3.6. And I don't expect another core developer to start applying patches to 3.5.
Hence my opinion that I should use hg rename, to keep continuity of revision history as much as possible, and that I should push a complete rename patch as soom as possible.
Two things that I *will* apply to 3.5, if and when possible, are fixing the third regression (Idle preferences not opening on Linux with tk 8.6, #26673) and fixing freezing of Macs when clicking on a completion window (#15786).
To me, this issue is a start on paying down IDLE's technical debt.