My Volla 22 has now received version 13 (Halium 13?). However, I would now like to replace the Volla OS with Ubuntu Touch. Will this work with the Volla/Android version 13? Or how can I reset the phone to Volla OS 12? Factory reset didn't work. According to the instructions, Halium 12 is required for installation.

Just a few basic thoughts from me:
Many mobile OS have disappeared because there simply wasn't much choice in apps. UT also doesn't have a wide distribution because there aren't many apps available. My wife would never switch from Android to UT for this reason. She simply lacks the little "helpers".
Therefore, it is important that every app, no matter how small, should be gratefully accepted. Every developer should be encouraged to continue to work for the community in his spare time (thanks to arubislander). His intentions should not be questioned. It doesn't have to be the huge "you-can-do-anything" app. Even the smallest app strengthens the OS. That's the only way UT can become something big.