See this post including the Amanda Readme this links to. You can install Amanda from the openHABian menu.
No need for that. Just service openhab2 start next time.
No I don’t think so. Use apt-get purge openhab2 openhab-addons in shell, then re-install either using shell or openhabian menu.
I was referring to openhab-cli backup/restore shell command. Read up on it.