If its at least somewhat recent hopefully those affected still have paperwork for any property ownership transfers. Finding proof of property you bought decades ago would be a huge pain.

It depends. In my country the online land register data is just a copy of the physical land owning certificate. The physical certificates (1 for the owner, 1 for the local government, and at least 1 more for some document keeping agency) are the source of truth.

Quite likely the opposite: a few weeks ago a ransomware attack halted ~100 hospitals' management systems in Romania, and the cybercrime defense unit just disconnected all hospitals and had the local admins rebuild from backups and paper trails. So I'm quite sure that all public administration IT admins have been running drills and probably have up-to-date backups.