Nice!
Don’t know about this. This needs to be discussed with Qubes.
Also don’t know if or how Qubes implementation of sys-gui (GUI domain) will interact with this.
This one too.
If needed, then Whonix-Workstation could ship a systemd drop-in file for Qubes gui-agent which sets this environment variable. An alternative mechanism could be a status file.
Otherwise environment variables could get messy. (How to set global environment variables as a Linux distribution? - /etc/environment.d)
But let’s see. These details can be discussed later after getting feedback from Qubes.