The two main points that seem to be being discussed here are the redundancy and the prospect of BMI remaining a subsidiary.
Having been involved in a BA subsidiary in the past, there are a couple of things I can throw into the pot for consideration.
Once this becomes the responsabillity of BA, they already have existing well tested methods and agreements for dealing with this sort of thing.
Firstly, they will not be able to run a subsidiary company with any aircraft type carrying more than 100 pax unless this type is not the same type that in already operated by existing mainline crew (long established union agreement)
Secondly, they operate a system that makes staff redundant in the exact way that BMI seem to be proposing already ie, seniority only applies within your base.
You are afforded protection in line with your joining date and fleet, but only at your existing base, so if they close your base, or dispose of your fleet at your base, you may only bid for what may be available elsewhere. If there is nothing available or suitable, you are redundant. Been there, seen it, got the tee shirt!
I have no doubt that BA will be happy to look at employing most if not all the BMI crews, but if they are forced to apply individually, they will only join as new starters with no seniority, and as new FO's.
You have my sympathy guy's and girls, it's an awful place to be.