Yeah, wanted to add that I understand why you often don't speak up when DD is rude, Lynker. As Lillie says, it might not really help.
But if it gets too much, sometimes, is it possible for you to just leave (if you're at her house, for example) or make some excuse to end the visit if she's at yours? Or end the conversation if you're on the phone? Maybe you could even say, "I see you're upset now, so I'll give you some space. Let's talk later." I don't know. You know your DD and have more of an idea of how she might react to that than I do.
Heavenknows, I've noticed a change, also. I think we used to get mad at some of the same things, but we were less likely to be rude to or CO the offending mother or MIL so completely. We might cut back on contact or limit the types of contact, as you did, or get into arguments. But most of us still drew the line at actual rudeness - I would never say the things to my mother that my DDs sometimes say to me, no matter how tired or rattled I was. And most of us would never CO a mother or MIL so completely as some of them do today.
I'm not sure if the change signals a lack of respect though or just a lesser tolerance for certain behaviors. (I'm NOT saying that the OP has done anything wrong, just speaking in general.) Ok, of course rudeness is disrespectful, but isn't it possible that, in some cases, it just means the DD or DIL is fighting more forcefully against the DM or MIL crossing any boundaries?
I know that you (general parent) can avoid offering unsought advice, as you do, and still have your DD go off on you at times. I rarely offer unsolicited advice either and yet, sometimes, one of my DDs will go off on me, just cuz she's in a bad mood or whatever. So I know where you and the OP are coming from.
But I'm wondering how you rebuilt your relationship with your mother, unless she cut back on the "demanding and overbearing" behavior. Did you let her know what was bothering you? Or find a way to stave her negative behavior off? Or did she just change after having experienced limited contact? I know this in not what this thread is about. But maybe your solutions with your mother will help Lynker find new ways to deal with her DD.