Some thoughts on the 'exorcism' angle.
My only knowledge of the religious exorcism is from things like History channel documentaries of how they were performed hundreds of years ago, and a movie I watched some time ago about a priest who performed an exorcism on a late teen girl who died, and the priest was put on trial for murder. As far as I know (not being Catholic) the actual religious rite of exorcism is shrouded in quite a lot of secrecy...which means that there are multiple things that could be meant by 'exorcism', you could be talking about the modern rites, performed by a priest, ancient rites performed by a priest (often on someone who was mentally ill) or you could be talking about pseudoscience and dramatic movies that are inspired by the concept of demons and exorcism but bear little relation to reality.
A lot of the hundreds of years old stuff on exorcisms (performed by priests) would probably include bodily restriction, and I have a vague recollection of special chairs designed to restrict the person during the process, so they couldn't hurt other people, and maybe also to prevent them hurting themselves? That seems to be a common thread, that the demon doesn't want to leave and there would be vocal and violent outbursts as the demon is forcibly driven from the body, and during that time the individual is liable to get hurt....ideally they do need restriction and/or in an empty room with nothing but cushions.
Leaving aside the question of whether any exorcism could be 'real', MM wouldn't have been performing a 'real' exorcism as a religious rite...for that you'd want a priest in charge anyway! So he'd be led by pseudoscience books and movies. I imagine that if he'd planned an exorcism in advance that he'd want someone else there as a witness, this doesn't really match what RB has allegedly claimed happened.
So all I can theoretically imagine is that the baby's crying and doesn't want to go to sleep. MM, in some twisted rationale, says to himself, "it's not Bella causing all these problems, it's the demon inside her. Right, I'm going to deal with this once and for all" and decides to force the demon out by squeezing it out? The more Bella screams and struggles, the more he thinks this is the demon being released. And then she goes still. This would be a spur of the moment thing that kind of matches up with what RB has allegedly alleged. But imho it's massive conjecture and could only lead to an insanity plea from MM *if* he decides to change his story from thinking Bella had been taken by social services and decides to admit he did it. On the other hand, insanity means something different in legal terms than in other ways it's thought of, someone could perform an 'insane' act from 'insane' thinking and still be culpable because they knew their actions were likely to cause death as the result. And according to RB, MM was extremely blase afterward, simply saying that "it was her time to die".