Cool. I've used Odin a lot, but never realized you could use it without root. I think the locked bootloader of the ATT Note 4 wouldn't allow a flash though so I would assume us ATT owners would be stuck with Lollipop if we upgraded. I'm not sure though.
I'm not sure either but my hunch is att models cannot downgrade like you said. I'm curious why though because im not sure that Odin requires a bootloader unlocked.