You could end up with all existing bmi flying only being available to bmi pilots.
Possible!
As it changes to new long haul destinations then they will have to have that too.
That doesn't automatically follow. Besides, I suspect a significant number of bmi pilots would prefer to head for the bottom of the BA MSL at the first opportunity rather than mark time at the bottom of a shrinking fleet. bmi might not even have enough pilots in the bubble to fly all their previous work!
You can't reduce their 6 week roster notice (TUPE)
Can't you? TUPE doesn't lock everything in a previous contract in position, ad infinitum. BA have plenty of scope to make justifiable changes. They'd be scrutinised, but TUPE doesn't prevent change. If 6 weeks notice of roster was enforced then BA could simply take a leaf out of bmi's books and just publish a 'roster' six weeks in advance then change it every day!