Hello,
If you have multiple DEs installed, then you log out and chose the other DE and log in. Not quite recommended but possible. With multiple users you simply switch users and each can chose their favorite DE. LightDM, SDDM or GDM offer that possibility already.
TimeShift does that already if the user sets it. You can use timeshift-autosnap available in the AUR; and if you use btrfs you can make snapshots:
There is no such thing. Services yes, and particular DE settings/widgets/addons … but most applications are DE independent. They might be GTK+ or Qt or whatever, and follow the theme of each, but will work regardless the DE or WM.
This is quite unrealistic and idealized scenario …
Each DE has their own development path and they address their own issues in time. Conflicting configs between them will not be addressed by anybody else but the user, by knowing what they are doing in multiple DE environment.
Manjaro provides ISOs for each DE the user likes. Install it and use it. Trough Architect you can install multiple DEs, but will be users responsibility to deal with them.
You will do that a lot, till you find one that better suits to your workflow and needs.