I think you might not have recognised that there are two separate issues. If balpa chooses not to represent a group of its members simply because 3300 ba members won't like the outcome, then that flies in the face of their mission statement "to represent all uk pilots".
I do agree though that bmi pilots expecting to come in with their doj intact in respect of seniority is a non starter. It is also a non starter to expect those same bmi pilots to come in at the bottom to start again, especially when that position is not supported in law.
Balpa members all pay a substantial contribution and expect to be represented in a consistent and fair manner.