You say this machine is headless. Is it at a remote location? If not, is it feasible to connect it to a monitor an keyboard for a few minutes? If so then you could logout, switch DE, and then log back in. That would hopefully set the DE you prefer as user default.
If that’s not possible, then some of the solutions discussed here might be applicable.
Yes, you can use it fully offline.
To back it up I believe you’d just need to backup your
.pass
and.gnupg
directories.I haven’t used keepass, but the entry from the archwiki should give you a good idea of usage, and it also lists some helper apps: https://wiki.archlinux.org/title/Pass