"From the above it is clear that what Boeing fears is APU pressure going into the engine, not the opposite.
That's why it seems that increasing power would help preventing this problem."
It sounds to me that whatever pressure is greatest, poses the threat. The APU on a Classic puts out fairly high psi with no load, posing a threat to the engines. When the engines throttle up, the pressure coming from the engines may be a threat to the APU.
Figuring out which poses the greatest threat to what on an NG may be a bit trickier to analyse as the APU puts out bleed pressure according to demand (Pack LO/HI flow, ECS demands, Main Engine Start, etc..).
Any help?
Cheers.
Q.