That would however leave balpa exposed to a breach of contact with every none ba member in the country. A dilemma that must surely be weighing on their minds.
Unlikely. It has nothing to do with non-BA BALPA members other than those in BMI. In practice, BALPA has to represent the best interests of it's overall membership (BA and everyone else) therefore it is unlikely to take a position to avoid any issues from either side.
Remember however that if anyone from BMI challenges the BA seniority system, it will actually be BA/IAG who they take on, not BALPA. If you think you need deep pockets to take on BALPA, you ain't seen nothin' yet.