hello,
pep 397 describes a "Python launcher for the Windows platform.
    A Python launcher is a single executable which uses a number of 
    heuristics to locate a Python executable and launch it with a
    specified command line."

Problem: that affects only the "open" file action behavior, and not the "EDIT with idle" file action behavior, and then it is the last installed IDLE who wins.
the problem is not the IDLE changes themselves, but that one version's IDLE can only run the edited script in its version, and ignoring the shebang.

Could the windows installer install a third executable (for ex: pyi.exe)
which launch the correct IDLE according to the shebang line of the edited file?
Or maybe it should be done using "pyw.exe /edit <filepath>"...

Than you for reading this and have a nice day/evening!