"I have presumed the the appropriate selections are made for other systems."

As far as I know, there is nothing in IDLE itself to do that.  As I noted earlier, there is a kludge in the Mac Makefile to edit config-main.def and config-extensions.def during installs of OS X framework builds.  That doesn't help non-framework builds on OS X nor any builds on other non-Windows platforms.  They default to the values in the checked-in versions, which default to Classic Windows.

"Can any of the editing of Mac/Makefile be moved into runtime?"

If you mean move the editing of config-main.def et al into IDLE itself, sure. That would address the issue of defaults for non-framework builds for OS X and could be extended to other non-Windows cases, assuming it was desirable to change the current defaults (I don't know if that is the case).  On the other hand, that would only help when a user starts IDLE without existing .idlerc/*.cfg files, e.g. the first time the user uses IDLE and the first time was with a newer version of IDLE that had this feature.  Presumably existing .cfg files would not be edited.

"In 'all versions by default share the same user configuration files', does versions mean 2.7, 3.4, 3.5 on one machine? If so, the statement describes a great feature that I would not change."

It means all versions of IDLE, not just the latest releases of branches undergoing active maintenance.  We can't assume that the user is just using those.  For example, various versions of IDLE are shipped with many OS's or by third-party package managers; Apple ships three versions of Python - 2.5, 2.6, and 2.7 - with the current release of OS X.  That means all of those versions of IDLE and any others (newer or older) that the user has installed are all sharing the same configuration files.  Is it reasonable to guarantee compatibility across all of them?  And does it make sense to provide the same list of recent files for all versions, especially across Py2 and Py3?  I don't think there is one right answer to these questions but today users have no choice.  As IDLE continues to be enhanced and changes are made, the probability of possibly catastrophic incompatibilities will no doubt increase.  I don't know if there are any today.  I think it would be better to eliminate that risk.  There certainly are other use cases, i.e. where home directories are shared across machines of the same OS type; I think the above considerations apply to nearly all of them, as well.