Note that in older versions of Debian the su command came from the old shadow source package, but Debian 10's su comes from util-linux source code and has different semantics.
Depending on how exactly you're switching to root, you might now be getting /sbin and /usr/sbin omitted from your PATH, which would explain the shell not finding the LVM tools.
Well, this is nice, posting an answer to your own bountied question. When I say nice, of course I mean not.
The reason why LVM fails to create the snapshot volume is because of a glitch in udev version 239 and 240 discussed here. There it says that LVM volumes are not properly initialized by the udev. When this is put together with information found in this ...
Should I be using /dev/mapper/ubuntu--vg-root to mount my root image so that the mirror stays intact?
It looks like, even if quite broken, your system launched the LVM stuff and your PV/VG/LV has been correctly detected.
So just mounting your LV will work and keep your mirror synchronized.
What you have inside the LVM are LVM logical volumes, not partitions. Logical volumes (LVs for short) cannot exist outside LVM volume groups.
Transforming a LV into a partition without moving the data would require deleting the LV configuration and reducing the LVM PV size without overwriting the data blocks of the former LV, and finally adjusting the ...