How often did your DF visit his Mum before his Dad died? When did he start visiting her morning, lunch and evening? Has your fMIL stopped eating? I am wondering if he is worried about her eating and has upped visiting to make sure she eats?
Perhaps as the wedding has got closer, his Mum is struggling more, especially at the thought of her husband not being there. I imagine this will be the first big family occasion, which she will have gone to without her husband.
I don’t like that he shouts at you and threatens you with cancelling the wedding just for asking him to come straight home in the evening. That is a major red flag for me. Has he always been dismissive of your feelings and aggressive/ confrontational?
Would he listen if you asked him to come home to spend time with/ bathe/ read a bedtime story with your little one on the evenings that he works the early shift? Then he can go round to his Mums on the late shift, as your little one will be in bed early anyway.
I still don’t get why you are being given a hard time because your period made you ill. I am bedded with my heavy period today, I am literally wiped and would not be safe to look after/ make dinner for/ bathe etc a three year old. It reminds me of my PSE teacher, who was adamant that women only lost up to three tablespoons of blood during their period. I told her that was the first five minutes for me and she refused to consider that she was wrong! Just because for one person, periods are a breeze, does NOT mean that someone else isn’t floored, in extreme pain and drained by their periods! I personally think her dF was a complete asshole to leave her on her own with a three year old. If she left work unwell due to her period it must have been pretty severe! Leaving her unwell with a preschooler was out of order. What if she had just had a c-section and he left her in charge of a three year old/ newborn? Would that have been ok?
I know that eight months is not long grief wise, but it is long when the OP is picking up/ feeding/ bathing then putting to bed a three year old on her own every night. It is OP who will have to hear her little one ask where Daddy is, or see her baby crying because they miss him. That is not easy (I know, I have been there, my dh would work away for 2-3 weeks at a time, but at least he was working) especially when you know he could be home and spending time with you and your little one.
Op is clearly meant to spend every weekend with her MIL too, when is she meant to spend time with her own family? It is heartbreaking that op’s Mil has lost her husband. I can only imagine how lost she must feel. But there is no way I would allow my son or daughter to spend all of their waking time with me, in detriment to their child and partner. This is ridiculous and not going to help anyone if it continues! When is MIL supposed to learn how to be herself if her ds is constantly there?