I know this thread is quite old, but it helped my a lot last week, when I was changing the rendering of my TD game from 2d to isometric. There is not much info about isometric sorting on the web, and if so it is mostly about the more trivial cases with no fluently moving units. I documented my final implementation as well as the used resources here. Maybe someone else doing research on this topic might find it useful!

Thanks for the in-depth info on this board!