Now, apart from my earlier suspicion re the husband, I'm thinking that it's Grace's half-brother that could be the villain. He said to M's husband that Grace told him about te affair - but we don't know that! He was quite off with her when she tried to speak fpr him and to help - quite agressive I thought, so his story about her sharing everything with him is questionable! Out of people who knew were both women lived he (apart from the husband) is one - he could have followed Grace or detected somehow re the affair - and knew the addresses of both as they were co-workers. The scene were he was concerned and questioning H - that's a classic thing murderes do. The motive - poss something to do with the didgy company dealings/mother/whatever - it wasn't for nothing that that whole thread is shown, re business.
There may well be TWO murderers - one who always used the bags over heads (poss the original suspect who M suspects all along) and the other one who copies the style to trick the police. I m sure the blonde's murder was copycat in style to mislead M and the police. Whoever did it, took BOTH women in the car, but M was knocked out yet had some flashes of memory through that blackout or being knocked-out.