Faulty mains main breaker

[mmagin]

Lessons: yes, faulty old breakers can trip too easily.  Also, multi-pole "handle-tie" breakers suck.

I don't understand, you mean that a one-side overload tripped both sides (or all 3) of the upstream breaker?  That is by design.

No, only one pole tripped, despite the handle-tie, leaving one side of the split phase live