This thread popped up in new posts, so decided to give it a read. Did you host with this guy for 20 years? The entire time locked out as you give your reason for never having your own backups? I noticed in this thread you like to brag on being a well polished "SysAdmin", yet you never kept your own backups over the years in an external drive you can store in your desk draw? I ran my site for six years, three years using xenForo and I have a back up zip file of every month of those 6 years saved in flash drives. If you never did this, unless you were locked out for 20 years, you can't put full blame for 20 years loss on this guy, you are guilty of the back up negligence as well. You should had some sort of back up, even if 5 years ago to restore from saved at home. I know to do this as a kindergarten teacher.
Why didn't you make a database backup then? There are addons in Resource Manager that allow you to make database backups without having to use PHP My Admin.
Same again, in 10 years you never made and saved a backup yourself? I can start to see now why your site partner put his trust in the host over you to handle site side things. Maybe this guy was told to lock you out by your partner whom you said was the one paying for the service as he didn't feel you were a very confident SysAdmin? I'm confused if host was so bad, why you hosted with them for so long and if no issues until near the end, why you didn't keep any of the backups while things were good with the host.