I am not completely sure what you are asking. Config-keys.def has 4 Idle Classic sections, one each for Windows, Unix, Mac (still needed?), and OSX. On my Windows install, the one for Windows is preselected. I have presumed the the appropriate selections are made for other systems. If so, your opening sentence 'provide different default settings depending on which platform it is running,' would be true already. If not, that would be a bug.

ConfigHandler.IdleConf.GetCoreKeys has hard-coded backups that probably work on Windows and Unix but not Max/OSX. I don't know if the 'Alt' to 'Option' replacement is applied to these are not.

If 'Alt' to 'Option' is insufficient for key bindings in config-extensions.def, I don't know what the answer is. Non-mac extension writers are unlikely to know what else to do.

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

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. If 'versions' means Windows Idle on one machine and OSX Idle on another machine, then the statement make no sense to me as we do not deliver user configuration files. If you mean multiple OS versions on the same machine, and it is possible to login as the same user with different Oses (I have no idea if this is so), then I could see a problem.