Nice article, and recommended reading - even if it only summarises the questions rather than providing answers. The possibility of performing a patch repair to allow for a ferry flight to Boeing's facilities for a more permanent solution is an intriguing one
I believe that somewhere in the process that the aircraft is going to return to PAE. Whether it be for some type of re-certification or to do a final structural analysis on the repair. Then clean and replace heat and smoke damaged interior parts. Just a guess.
Cost, I suspect there is some negotiations between Honeywell, Boeing, Ethiopian and the insurance company with Ethiopian getting some kind of guarantee of performance of the aircraft after the Boeing repair. I would doubt that Ethiopian would accept flying around a multi-thousand pound weight penalty for doubled panels to repair the damage on a new aircraft for the rest of its life.
So far other than the external pictures we have no real idea of the actual structural damage to the fuselage and the heat damage to the rest of the aircraft. We can guess from the number of containers, tents and scaffolding that this is going to be a major time consuming and costly repair.