Personally I don't think you're being unreasonable OP. I had this in my family. 'Son' declares he'll be abandoning his wife and children for a workplace girl with a drug problem ("she's in rehab!") Far from 'supporting' him, they were disappointed he was such a scumbag and remained fairly neutral. They offered help and kindness to the wife, their DIL, and the kids, and their son got civil chat but certainly no red carpet. It was really hard for them - he was their son and they really wanted to somehow drudge up some feelings for him, but what he did to his wife and kids was just unforgivable. The kids were traumatised (he simply walked out and didn't see them for a long time, then whenever he did, he'd drop them at Gran's without a backward glance) and his wife's struggled financially ever since. And I can totally see where they're coming from. They wanted to feel 'OK' about it, but it was so fucking vicious, what he did. They couldn't see past it. He wasn't the same, to them, anymore. They saw him for the unpleasant person he was.
Everyone's 'moved on', in a way, but really the wounds are still raw. They're so disappointed in him and now don't speak to him - they drifted apart, as people with different value systems tend to.
What a catch, crawling back to mummy! Yuck. At least some of them get flats of their own or move straight in with the other woman, which is fractionally less pathetic than sitting in your childhood bedroom while mummy cooks. Big baby. Where was I? Right, yeah, OK, so Mum has offered him a room and that, which is eyerolly but fair enough. I think playing happy families with the other woman is crossing a line though. You're the mother of her grandchildren, and unless she wants to be erased from their lives it's in her interests to be nice to you. Posting up happy-family pics like that is a shitty thing to do. Wait, did you say she posted pics up of the other woman, or just of the nuclear family?
Still, even if just the family, it's a shame. She's avoiding the topic with you, and if she's got no support to offer there's no point her coming round. One benefit of divorce is you no longer have to play nice with 'family members of a man I was once married to'. Dad can arrange any time for his parents to see the grandkids (believe me, he will. Frequently) - it is literally no longer your business unless you make it so.
YANBU OP. She could put her bloody arm around you and tell you she thinks it's proper shit. That's all you need - some acknowledgment. Not this singy-songy nonsense.