Old smokey,
While the ETOPs fuel is greater than the great circle fuel, in the case described, the engine out depressurised fuel in the last sector requires more gas than the increased fuel load for flying the ETOPS route and that and is the limiting factor.
It mostly depends upon the distance apart of the available ETP airfields and the winds at 10,000 ft.
Pilots flying these sectors are always concious of remaining within the required range circle and having the gas to get there single engine depressurised.
For the B763 you have been correct all along that engine out depressurised (flown single engine with TAS 370kts at 10,000ft ) fuel is the more limiting fuel scenario than the ETOPs fuel (flown on one engine pressurised at 415 knots TAS at whatever altitude you can).
I imagine that the same situation exists for all other extended range twins. That's why my first statement that the bottom line of ETOPS is staying within the appropriate range circle of an adequate and suitable airfield.
I agree it has been interesting and thought provoking. Good topic.
