Old Engineer:
2. Starting an engine can apparently be accomplished by one of the two generator/starter on the engine.
4. Loss of a main turbine takes out two generators. How realistic is it to call one generator a back up to the other? The APU generators are the back up, hence the importance of being able to start that system aloft.
2. My understanding it that even though the two S/Gs are identical, only one of them is wired to be used for starting via the APU battery. It is not selectable. So you have two generators, but only one starter, at least when only battery power is available.
4. I have seen the claim seriously made by Boeing that the APU is needed for ground operation only and that flight safety without engines can be maintained well enough by the RAT. According to this argument, the two APU generators are only important for delivering enough power for comfort and convenience on the ground and for starting the main engines without needing an excessive number of ground power units. I do not know whether just one of the generators is sufficient for that or if both are needed, going along with your point #1.
Yet, at the same time, Boeing
PR is going strong pushing how safe the plane is with six redundant generators. Trying to have it both ways, I guess.