You should be able to simply add the /4kmovies path to the /movies library, and assuming you do not have any non-standard naming, plex should automatically match and stack duplicate movies (they have the number in the corner).
generally, yes, I think there may be some differences between various client though.
assuming you have sufficient server and hw gpu support, plex can now tone map the HDR files to SDR for non 4k/hdr clients, so you may not need even need to keep the lower resolution files anymore.
Excellent. Both directories use the same naming convention.
Yes Ive noticed that. I have an 2060 doing my HE transcoding, downside is 4K transcodes use 4x the amount of VRAM so I try and keep 4K transcodes to a minimum. But it’s good to know it’s available!