Installing multiple distros at the same time would cause issues because of additional software most DE’s come with (image viewer, …). But yes, it’s possible to switch DE by uninstalling the desktop package group and installing another quite easily. Especially with btrfs snapshots it’s simple to roll back.
Yes, it’s possible to rollback with ublue but that won’t roll back changes in the home directory. So if you switched from Gnome to KDE and then back to Gnome the additional configuration from KDE might conflict with Gnome (especially theming breaks easily).