As I understand it IAG are buying BMI before it is merged with BA. Between the purchase date and the merger date BMI would continue to be a separate company within the IAG group.
Should the number of slots that IAG aquire with the aquisition of BMI be less than anticipated, due to EU pronouncements, then any reduction in personel and aircraft required to service those reduced slots could be made before the merger with BA i.e. all reductions could be made from within the BMI workforce without any reference to BA.