I was reading your post few times to make sure that I would not miss any part of it. I still don’t see how your theory can be correct.
Do you have a link to article or document talking about this issue?
I, specially don’t get part about” fault” traveling from source to transformer and back.
Circuit breaker reacts to over current flowing through it’s bi-metal element. It does not care if this current travels through neutral or grounding conductor.
Breakers are electro-mechanical devices. They are not fail proof.
I have seen 20A breaker been tripped by 15A current.
I also have seen 20A breaker holding 60A for quite few seconds before going off.
In this example where 200A main tripped before 50A, it is possible that 50A breaker was faulty. We have to remember that 200A panel was carrying other loads from the house so it needed much less then 200A to trip.