make sure the RAID is in good working condition.

Can I use the repair file system function in these conditions,
without causing data loss in raid?
Yes.

If NAS logical volume (Lv0) is unmounted, I lose data on raid?
No.

Can I backup my configuration using the USB Demo and restore it later
if I have problems with DOM?
Yes, if the system you are restoring to is the same version or newer.