I still think there is a 5% chance the "incapacitated" theory has merit.
What if the captain forgot the code to the door? Heck I draw a blank on my pin-code sometimes which hasn't changed in 10 yrs.
Does it happen that Crew forget the code and need the other crew member to help them out i.e. open the door to let them in?
If the F/O felt he was losing consciousness and his last action was initiating descent because he suspected hypoxia...... but his unconsciousness was caused by a medical event.... and the captain didn't have the code to the door....
I don't believeCabin Crew knows the code. That would be a very easy hijack target.