Q for the experts. When BA took over, did it assume the liabilities of BMI?
That is usually the case and if so, a BMI ticket should mean that the pax gets from BA whatever it would have received from BMI (legally). This would, of course, not extend to routine courtesies which may have existed in the past.
On the other hand, it may well be that there were specific legal exclusions, approved by the respective governments, with appropriate notices to pax (as consumers), relieving BA of the liability.
You really need that information in order to figure out whether you can make a demand or an "ask."