Regarding calendars and addressbooks: calcardbackup is able to recover calendars and addressbooks, as long as the data is still in the database.
EDIT: there is also an occ-command to move a calendar from one user to another. This example moves the calender personal from user dennis to user sabine (I don’t know though whether that will work, if a user no longer exists):
sudo -u www-data php occ dav:move-calendar personal dennis sabine