nl backseater
I think your questions are exactly on the right track and am willing to bet a Canadian dollar the NSTB findings will reflect the broken stud was related to the amount of times it was taken off and replaced , and how the torque sequence was applied.
Any gasket mounted fixture (oil filter) has an inheritant flexability at the mounting base because of the (rubber) seal.
More of a problem with a 3 point fastener system. For example if the first fastener was run to it's maximum depth , even just finger tight , and the next 2 were properly torqued to spec it is possible to exert 10 times the stress on the original fastener because of the inheritant rocking movement and leverage of a 3 point flange.
Systematic torquing each fastener in equal increments would avoid this , but is it always done that way? I would guess not sometimes.