Airline SOPs state:
Unless those SOPs are Eagles then it is, I fear to say, quite irrelevant what any other SOPs may say.
Even though the cargo door is part of the fuselage structure (so is a cabin window), it will not come off if the a/c is flown at circuit speeds.
How do you know that? For instance....
Is there an Abnormal or Emergency drill for flying a circuit with the door open at or below a certain speed. What would be, if any, the effect of flap selection on an open door and also--how far open?
The proof is in the pud: the said a/c landed back safely.
That is a big red light of a non sequitur I am afraid to say.
Just to clear up two things. I have never flown the B1900 and I am not a hero.