Touch off, OK button greyed out

The Axis touch-off allows for any valid G-code expression, including the use of parameters. (So you should be able to type [#<_x> / 2] for example, to save typing the current X position)

Given this, "bad number format" is not a bug, but "file not found" is.

github.com/LinuxCNC/linuxcnc/issues/3563

What is the best way to move from 2.9.5 back to 2.9.4?

Uninstall 2.9.5

Manually download the 2.9.4 deb files using gdebi.

Or you can follow the instructions here:
gist.github.com/cgomesu/70b41cfde804d4a2981029f9f71f8908


Do I need to say use Linuxcnc as your package

What I did: In xfce you can go System|Synaptic Package Manager, search for Linuxcnc, mark linuxcnc-uspace. In Menu -> Package Force Version->you can select an older version -> apply.

Good advice, I tend to forget there's a GUI for stuff like that.