Is 2.6.5 on the Mac still compatible with 1.8.6 on Windows. If not, is there a timeline to realign them?
I would think not, which is why I've put 2.6 in a separate applications folder within my user account, and 2.6.5 in the main Applications folder. That way, any project that I collaborate on with my friend, I will continue to edit in 2.6; any project which for the moment is "For my eyes only" is converted under 2.6.
As to a timeline for compatibility, maybe LAP or Tiho_D will find time to comment. It's certainly going to become important as people upgrade to El Capitan. But what do I know?