You should be able to do all that from the GUI too at some point, even though it doesn't work on your phone yet (works well on Pro¹)!

2. Sound works out of the box for me. Maybe I needed to install pactl/pavucontrol, I don't remember. Then the sound from some applications can be adjusted with the hardware volume rocker, while some others' sound is unaffected and can only be affected in pavucontrol. I cannot firmly confirm yet if it actually works this way in some apps and not the others, or this was the whole session that responded to the volume rocker some times and not others.

3. I never observed that, but don't attach often. I need to test if I can reproduce this.

4. You can export your Onboard settings using the dconf-cli package to run:

Find the coordinates setting and fix it in a text editor, and then re-import the modified settings with: