Inetdog
I think you are confusing Engine starter/generators and APU starter/generators.
Only one starter is required to start an engine, it's slow (3 mins) but it will do it. Normally both are used but it needs both APU generators supplying or three ground power units.
The APU on the other hand only uses one starter at a time. One 'hard wired' as it were, to the APU Bat Bus, the other from one of the main distribution busses.
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.
Essentially correct but it depends on what you mean by 'redundancy'.
The APU generators will need to be operational if the a/c is dispatched with an engine generator inop.
The statistical evidence from many years of ETOPs OPS is that the chances of a single engine failure mean that the APU can be inop for dispatch.
However if you dispatch with one engine generator inop the chances of losing the other (generator) increase, therefore APU (generator) must be operational.
This is standard ETOPs policy.
Is the 787 any different in this regard? No. Unless of course you are mid atlantic on an MEL with one gen inop, you lose the other engine and then the APU battery gives up, shutting down the APU.
In the past, all very unlikely, but with the 787 a bit more likely. Too much more for my liking. An independent power source for the APU is a must until these batteries can prove their reliability. The main engines will keep running if you lose all electrical supplies because they have a PMG. Why not on the APU?