Thanks mofi for answering and confirming my suspicion. I indeed have multi monitors (3), and part of the point of my question was to achieve an arrangement of editor windows in different sizes at convenient places on screen without having to spread the main MDI window to some huge size thus obscuring anything else that's running. That's after all the logic of having floating "views". It's not clear to me why "views" have better floatability than "windows" -- though UE is not the only app I've seen that has this restriction.
That said, on experimentation I see that I can maximize an editor window, and get rid of the clutter in the surrounding MDI parent window, at which juncture the full window is pretty economical -- then use Open New Instance, and voila -- the effect of multiple free-floating edit windows, more or less. That in combination with split is working a bit better for me. However, I believe it means that if I need to use various toolbars or views, I'll need to open them up for each of these windows (as they're separate application instances), and that starts to defeat the purpose.... so I'm not quite in utopia yet, I suspect!