Ok, I can answer my first question myself: yes, backing up /home/phablet/ is enough for all the normal data, incl. SMS, contacts etc. Even all the 2FA entries in Authenticator survived.
(Got a new phone (Volla 22) today, installed all the apps, rebooted, dumped a tarball of /home/phablet from the old phone (FP2), rebooted again just for the sake of completeness and et voila.)