I don't get the whole thing about Mrs. Gallagher. Why were the kids afraid of her? What was she doing to them? Why did she take Monica to her house? Why did neither the cardinal nor the bishop seem to care that she was recording confessions? Why did the priest not confront her with that when he was called into the cardinal's office on her accusations?
shareShe was a junkie. She was abusing and getting drugs from the kids.
The Cardinal, Bishop, and the Priest not confronting her on the recordings are more commonly known as minor plot holes. Had Slater mentioned them while she was in the office, then there would have had to have been a "real" confrontation. But, since he mentioned them after she left, they could be brushed aside to further the plot along.
This was an interesting plot, well acted, but there were quite a few obvious details that pointed to who the killer was. The main one being you had an established star in a minor role.