Creampuff.
What I followed was the complexity of what you wrote. I certainly don't claim to have an answer from what you wrote.
As far as to what happens if no one "volunteers" for the overwing exits, I expect either the operator would have to find crew for the positions or pax numbers would have to be reduced to a number specified for the front and rear doors only. I have never seen that figure, as all MEL/DDGs that I have operated under assume only one exit blocked out and a comensurate reduced pax number to meet the 90 second evacuaton ruleThat is why I query if in reality the pax are assigned a duty. If they weren't there the flight goes no where.
Another one to consider. If a test engineer is assigned a duty on a pax aircraft and is operating onboard approved testing systems are they crew or pax?
Wunwing