Grace's mother is definitely a cold-hearted bitchy character. Nasty to Henry with her cruel put-downs, and she is ALL about greed/money really, and to some extent her DP. Grace wanted to put a spanner into the money-grabbing business deal. - though imo she was trying to prove her status and maybe wouldn't haev gone too far. And then she suspected her DP was attracted to Grace (probably seen his car parked at G's house - whattever he as doing there) - she could have flown into rage and hit Grace without even meaning to kill her. The if G dies as a result they could haev put the bag on etc to mislead police (but it wasn't the mum dong that - whoever helped her, I hope not Henry, but he is the one who knew about the murders by visitng Marcella's house).
I think she is upset and grieving about Grace, she's just very controlled - but I don't think it's as important to her as the business/money, she hasn't softened a bit since the death, still greedy and unscrupulous, and once more nasty to Henry. She may still crack up but her demeanour now is 'it's all shit, the only thing I can control is my business so to hell with people'. Hardly a happy face, is she!