Thanks @DLane, I missed that one.
I am a programmer, work with programmers. The significant and primary reason that this would be complex is due to the stated, “more sophisticated weather intelligence features across schedules”.
What about allowing multiple controller merge only with Fixed Schedules? That is what most of us with multiple controllers are using anyway. Grey-out the “intelligent” features with a caveat that explains the feature is not available for multiple controllers once merged. Schedule data is stored on the controller itself. The iOS/Android/Web interfaces are talking to the controllers and affecting changes to databases anyway. I don’t need a dB in the cloud (would be nice of course as then you could implement intelligent features), just simple coordination of timing across controllers with Fixed Schedules. Let the app check for busy true/false for the desired start/stop time and if the app sees a conflict, warn the user. if the user persists and is okay with two zones overlapping, notate it in the app and display the warning.