Wow, 92 years old and so independent is impressive. At 72 years old I had C3-7 fused but it was done anteriorly through my neck by neurosurgeon. The recovery was a breeze compared to my L4-5 posterior fusion but also much less weight bearing. I wore the neck brace the first night but after that only when I was out of bed for the next six weeks. I was only in bed at night to sleep. I was supposed to stay two nights in the hospital but begged to go home after one.
It is right to be concerned about cognitive risks of anesthesia for elderly. I am a retired nurse and the best way to combat the confusion elderly may have is to get them out of bed. Sit in a chair, stay upright and walk, walk, walk, even if it's just in the room.
Since your dad is not needing C3-4 & C4-5, maybe the posterior approach is best. I would definitely get a second opinion for necessity and anterior vs posterior technique - anterior may be faster, maybe slower, just ask. Write your questions done so you don't get rushed or forget.
I was given a pain pill & a muscle relaxer postop for pain management - I was really dizzy. I asked the nurse to skip the muscle relaxer and what a difference. I told my doctor those pills weren't good for elderly or maybe cut the dose in half. I just didn't take those muscle relaxers any longer.
I hope you get the answers you need and your father does well no matter he decides. He sounds amazing.

Kindest regards,
Alexis Casazza