No legal challenge, that's funny because it's ready to go at the sniff of ignorance to current employment laws which in recent years have changed significantly.
If you think that 1) BA will endorse a solution that leaves them open to legal challenge and/or 2) that you/BMI pilots/BMI-CC have the financial resources to take them on even if they did, you're being a bit naive in my opinion.
Just like if BA merged into the Iberia pilot seniority list. They wouldn't expect to be shoved at the bottom like the BACC are planning here would they!?
If Iberia bought BA, and SEPLA wanted to stick me at the bottom of the Iberia MSL but offered me protections to maintain my current career prospects, I wouldn't have a problem with that. In fact, if BA was on the brink of bankruptcy and Iberia was merely interested in slots at LHR, I'd be delighted with that outcome.