Ah, OP, I feel for everyone in this situation, and understand your fears, your DHs reaction which is basically his inner child having a meltdown because naturally he's lost one parent and wants the remaining one to fulfill her scripted role as caregiver and to put him first. But him being an adult, the practicalities of that can be difficult to establish. Essentially he's missing both his parents, and angry because one of them isn't behaving as expected.
I'm 55 and was widowed 2.5 years ago. My DS is an adult and not my DPs son, but had a pretty close relationship with him, so it's different. I've been a hot mess tbh. It was unexpected death so the anger phase is still ebbing and flowing. I'm not going to bang on about me, but I do have some general observations about widowhood.
It's a paradox, especially if you are perceived to be part of a devoted couple. Simultaneously you are expected to Queen Victoria through an unspecified number of years (or even throw yourself into the grave after them in some eyes) while also bravely getting on with your life, not being needy and behaving with appropriate decorum. You must also be mindful that you are not the only person mourning the loss, and continue to support everyone because they "mean well". Which they probably do, but the awkwardness of death and grief can unleash some spectacularly bizarre behaviour in people close to you. Which has to be excused. However, going off piste as a widow is less acceptable, and frankly irritating or embarrassing in some eyes.
My late DPs DM has dementia, and has been bedbound in a home for about 6 years now, surviving thanks to the wonders of modern medicine. Watching her decline to that point absolutely slayed him. His inner toddler was there for all to see. We cared for her at home to start with. It was not pretty, and I don't even want to imagine what it's like for a spouse going from equal partner to nurse in what can be a shockingly brief period of time - it's devastating for a child if any age as it is. So I do have some sympathy for your DH, and for you stuck in the middle.
Thing is, everything has changed, irrevocably, for all of you in a short period of time. There is a strong emphasis in our culture of "getting back to normal" as soon as possible and resuming the old life just with one fundamental element missing which you have to "accept" and get over. But that's not real life, and that expectation leaves one feeling life is surreal indeed.
Personally, I think at some point, the DH could be encouraged to reach out and explain how much he is missing his parents - both of them. It might be that he's done a stand up job of being stoic and resilient for his DM and so she doesn't realise how much he's hurting, and if alot of their interactions have been about him judging her actions she might have withdrawn to avoid further hurt and conflict. It's always hard for children to see their parents as autonomous beings, even as adults, because if relationships have been good and solid, they represent unconditional love and security above all else, and the loss of that is devastating.
MIL is probably in a weird kind of hell right now. My escape was alcohol for a while, obvious not healthy or ideal, and hers is a new relationship. Whether it lasts or not, as a few have said, it would be sad if the estrangement grew out of all proportion.
On another note, it's interesting to see a thread like this, because normally people are worrying about widowed MILs being needy and overly dependent on family and asking for advice on how to encourage them to "live" again without their DH, and with less intensive input from family. Again, part of the wudow paradox I feel.
Ultimately it all takes time. Life has completely changed for you all, the adjustment period will be ongoing for a long time. What feels devastating now might seem less important down the line, and my final advice is simply to try and keep doors and communication open, if everyone does value each other as much as is suggested.
Sending you and yours all the love and solidarity OP. It's a very hard situation for everyone but MIL has alot going on, and if she's burying it in the arms of someone living and breathing, let her. Life is short, often painful and messy and yet it must be lived.