Hey op,
I seldom post on mumsnet but I have had a similar experience with a grieving MIL and wanted to say I completely empathise.
My FIL died in the midst of the pandemic from lunch cancer. We are fortunate to be a short drive away and both me and my husband had taken the last 6 weeks before his death off work to care for him at home as per his wishes.
The first year after was the most challenging time in our marriage. We decided for DH to take a 6 month career break to support his DM (who had lost her husband of 55 years) and process his own grief.
MIL was understandably distraught, but the level of projection, emotional manipulation and plain vitriol was off the charts that it ultimately led to my husband having a mental breakdown. In those 6 months, she insisted he go over every single day from sunrise to dusk during which time she would wail (please don't think I am being uncaring here, I totally get it) scream and use him as an enptopnal punchbag and just have my husband running round like a headless chicken doing and sorting whatever she needed. When he wasn't there she'd find excuses to call. On the odd occasion he couldn't make it, she would be on the phone trying to emotionally blackmail him using phrases such as "pleaaaaseeee, pleaasee, come over, i won't cope without you" or "your dad would be so disappointed in you not supporting me"
The idea was the 6 months career break was supposed to support MIL emotionally, but also help her transition to a new normal for her and establish a support community and routines.
My husband inevitably had to return to work but in all of this time, he'd never had the opportunity to process his own grief and the demanding of his time continued.
I had tried on a number of occasions gently suggesting to him this was untenable because I could see he was deteriorating. Things such as trying to reduce the length of his visits slowly so MIL still felt supported but that she gradually got used to spending time by herself. But alas, she resisted everything. Only DH would do and the only thing she was prepared to accept was him sitting in her house with her or taking her out on drives in the car.
That is until around a year later, I walked into our bedroom and the face that looked back at me I no longer recognised. My husband looked bedraggled, completely overwhelmed, exhausted, petrified. He looked locked in a state of fight or flight. Now consider me unkind if you wish, but my number 1 priority is to him, not MIL. He is my family. I told him there it had to stop or I'd end up with no husband as he would die from the stress.
He knew it himself too. So very gradually, he started pulling back. There were a number of rocky patches along the way. She stopped talking to him a few times when he wouldn't give in to her immediate demands, and she's still prone to trying to emotionally blackmail him. For example, she wanted him to do some weeding a couple of weeks ago but we had plans she she said "if your dad was here he would have sorted them by now" but he's so much more adept at deflecting or challenging it gently. Life is much better. They've settled on 2x weekly visits and a call each day (which my DH hates but he accepts as it helps his mum).
You have my sympathy and I hope you manage to get through this intact. I can understand that when somebody loses a partner of over 50 years, it must be an enormous trauma. But, I dont think that gives you the right to subject or make others endure the brunt of your pain to their detriment. My DH was grieving too, shouldn't his journey be just as important?
I hope the visit goes well and your MIL finds it supportive ❤️