Internal HD not mounted after upgrade: Encrypted: ERROR -69808

Even though you have now wiped the drive and are using your MacBook, I would strongly advise that you look at the SMART reports for your drive, especially if it is a traditional spinning HDD — mount: Input/output error suggests that your drive is failing.

For future reference, if your drive is failing, a tool like ddrescue will allow you to create a byte-for-byte copy of the readable data on the drive to another drive. The copy can then be inspected further to attempt data recovery without any risk of complete data loss due to the failing drive becoming completely inoperable.