We do not know yet. Most likely it will just track normal Manjaro stable.
We recommend containers for this. Distrobox specifically, it is an easy to use abstraction layer on top of Podman which nicely integrates it with your system.
It can diff, the current diff implementation was only added a week ago, I may still change it as the current implementation has no way of telling what image variant you are running, so it will assume the default.
The diff works by pulling package lists from the repos and comparing those.
https://download.manjaro.org/arkdep/test-manjaro-kde/8da5433c34d8633d686c0c6cdc8b0d7d0c77d0d5f7.pkgs
It does it using Bash, so it is not super fast. And I am sure the current implementation can be optimized a tiny bit.
They will continue living side by side.
It takes 10 minutes to build an XFCE config, someone just has to get around to building it.
Ideally you should not notice any difference.