Harris could serve 12 years.
Biden can stay in ‘office’ incapacitated, and VP is ‘Acting President’. See 25th Amendment....................
I don't understand the talk of the 25th amendment. That should be used for a very temporary condition (it has been used a couple of times when Presidents were under anesthesia for a medical procedure) or if a President were to become unconscious suddenly and likely to remain unconscious. In this situation the proper thing would be to force Biden to resign the office he cannot handle (and only has because of the stolen election).