I was wrong. I had admin mode unchecked in Properties - Shortcut - Advanced, but it was checked in Compatibility (not even sure how those two checkboxes are different?); so I was running in Admin mode when I thought I wasn't.
(I guess that explains why I was surprised that iTunes actually started.... as another forum discusses, there is a conflict in Win 10 v1073 between iTunes and the Bluetooth driver).
So, unchecked both Admin modes, killed BTTRAY.EXE, started iTunes, and voila, drag and drop works.
Ringtone back on my phone!
(the BT issues is here: Why doesn't iTunes launch under Windows 10 Creator?)