LEM
I'd say you about have it.
There's other things going on as well. In the NG there's a 110psi regulator in the HS regulator that acts as a "door slammer" in the event that the regulation system doesn't function. The High Stage (9th) valve is self regulated to 34psi by pressure downstream of the butterfly so once 5th stage gets to >34psi the HS valve is closing. If that doesn't work then when 9th stage pressure gets to 110psi the HS valve is commanded closed.
The PRSOV regulator has a 180 or 220 psi overpressure switch (depending on aircraft and operator) that electrically trips the system if duct pressure upstream of the PRSOV gets that high.
The NG APU now uses a load compressor so back pressuring the APU isn't the problem it once could have been.
These things along with what's described above all pretty much guarantee the ground engineers will remain gainfully employed for the remainder of their natural lives.
