This is the way I read the scene. DM is back to thinking mode just as he was in the thinking mode during the Anamika episode. So, his brain has started to work. At the same time he is upset that Santu used the POA and a lawyer to assert her rights. If Santu had run behind him or cried on the day he told her to do whatever she wanted, he would have been very thrilled and acted like a mahan and announced Santu can stay. Instead he finds a grown-up and educated Santu who can stand up for herself.
So, our DM is throwing a tantrum! He was not very angry when he was talking to Ba. It was more like "hey, she thinks she can outsmart me, I will show her" kinda speech. Thinking Santu is guilty is just more tantrum throwing, IMHO.
DM has lots of faults and he makes me soooo angry with him at times (as I was yesterday) but DM has one redeeming quality. He is very fair which is why, while coming down the stairs to talk to Baa, he thinks of the milk drowing scene and of Santu telling him that she has every right to stay in the house. His fairness, I think, accepts Santu's argument but his ego is not ready to let go. I think his sense of fairness will win in the end.