I think it's important to recognise that sometimes nightmare MILs are great DGMs, and vice versa. (And sometimes they nightmares or great at both.) My MIL has finally started to turn into a good DGM now that she's stopped trying to be DS's mother, but her MILing is beyond hopeless. I think that for us, the problems stem from a very different culture, combined with the fact that for the fifteen years before DS arrived, MIL spent the entire time constructing an elaborate picture in her mind of how it was all going to be for her, which was basically a replay of how it was for her and her DM/MIL. Basically she was going to be the absolute matriarch in sole charge of her adoring DGS, who dispensed her wisdom upon us all, and whenever I did appear, it was merely to do as she instructed, thank her profusely, and vanish again. Had I known she was imagining this future I would have nipped it in the bud at the get-go, but there you go. It's had fifteen years to cement itself so unsurprisingly, it has been so hard for her to let that dream go and adjust to the very different reality, in which I expect me and OH to be in charge of my DS. Unfortunately she was so overbearing in trying to make this dream happen with her DGS in the first two years that as soon as he could move, he started to actively avoid her. It was very hard for her, and there were a lot of tears, but finally, she's getting it. Now, rather than treating him like a little performing monkey and a trophy to show off to her friends, she's actually treating him like a person with his own feelings and wants. For the first time he is starting to ask after her and wants to spend time in her company.
But, she has made almost no progress in how to be a good MIL. She still believes that she is the matriarch and I should defer to her. Unsurprisingly, I categorically don't believe that. Where she has good advice I'll take it but largely her views on childcare are forty years out of date and heavily coloured by an extremely patriarchal culture, so, er, no ta. Inevitably, it causes huge amounts of friction because when I give her direct instructions about DS, she treats them as requests that she's entirely welcome to ignore. I tell her he isn't eating solids till 6 months. She tries to feed him solids at 4 months. I say sliced grapes. She gives him whole. I say no more chocolate because he's only one. She brings over four giant boxes of chocolate for him at Christmas. I say no big, loud toys for him. She buys him a massive indoor trampoline and a noisy massive ride-on zebra. And so on. So, she's getting the DGM thing to an extent, but the MILing is going to be a loooooong, hard slog.