Can't speak to my experience as I've I toggled anything differently. I am still on 20151212 too. I will say that my charging optimization is working on the tablet and failing on my phone as well.
As for desktop mode functionality, mines is (still) there w/o touching the Developer Mode flag.
One quirk/feature I realized is that I can toggle desktop mode via the Overview tab (swiping up to show all open apps). I find it easier/more logical to enter Desktop mode this way. Also, if you select a currently running app, it will launch that version into desktop mode fwiw