I don't think Lufthansa have taken over Brussels Airlines. My understanding is that they own 45% and have some kind of option to acquire more at some point.
Why couldn't Lufthansa retain Regional and use it to operate the routes in question to compete with IAG? They have bases at LHR, MAN, EDI and ABZ although would presumably only need a bigger type. Having said that I believe that Aberdeen has been operated exclusively by the Embraer for some time now, maybe to make a smaller loss than if it were to be operated by an Airbus.