Interesting contrast with the two letters -- one about a grown man (though not, as the vernacular would have it, a "grown-ass man") who can't cut the cord and a mother who is freaked out that her cord is being co-opted by a hussy.
For the problem of the man who is stunned by the idea of not driving 12-hours each weekend to see his parents, I'm not surprised that the "Maybe it's culture!" people popped out. Maybe girlfriend would have thought it was culture to start with and worked around that somehow (how? Maybe, "Here in America, boyfriends are expected to not want to be with their parents and to indulge in wanton sexcapades and partying on the weekend. Sound pleasant?" Or then again, maybe not).
But culture doesn't begin to explain this man-child. No, there are other things going on here, presumably none of which can be fixed or changed by yelling at the MC.
The project for the girlfriend: Sit down with the man-child and — without getting angry — lay out the two visions of the future you have described to Prudie. Explain why you don't want to be alone with the kids nor have to drive each weekend with them (and how does your MC spend his vacations? How often do you get to see your own parents?). Explain that you want his primary home to be the one you build together. Explain how it makes you feel that his parents are his priority, not you, and that their home is a place of relaxation and welcome, not what should be yours (plural).
He's going to get defensive, because that is what kids do. Keep asking him to listen to you and your pain. Counter his rationalizations with, "but I'm talking about my feelings." This will either cause him to implode in babyish tantrum or to actually pay attention to you. If he does the latter, you both have a chance. If he doesn't start paying attention to your needs, though, by cutting back on seeing mom and dad and spending more time with his boo, well, he's content to be making you sad.
My current SO used to visit his folks each weekend or every other weekend (a 2-hour drive, and to be fair, he has had for much of that time "extenuating family circumstances"). Then he met me and all that family time just wasn't as much fun anymore. Now he's there about once every other month — and as it turns out, that's fine with his family, too. So if your MC's parents pitch a fit (assuming he actually cuts back) then you know you're dealing with dysfunction and freakiness, and you'll have to decide how you're going to deal with that.
To "Mama Rosalind," well, ew. That sucks. You are being put on par with a hussy. A total man-stealing bitch from hell. And your pathetic, weak, manipulated dipstick of an ex is being complicit in your being knocked down to an equal with that total hussy in the eyes of your precious daughter.
What those two are doing is totally shitty and they know it. But if you call them on it, they'll be disingenuous. You know that already.
But at the same time you have to deal with them in a mature way. For your kids' sake. Trust me, if you are the more mature person in the next 20 years your kids will recognize it at some point or level and will adore you for it.
I still harbor the memory — of when I was maybe 7? — of my parents' separation. My dad was seeing some dumb bitch — also named Laurie! Coinkydink! — on the side while my mother grieved on her own but was way strong and positive with my brother and I. Although my folks got back together and 25 years later are still going strong, I remember how disappointed I was in my father, how selfish I learned he is. I still to this day have not brought up Laurie to him or my mother. I admire the hell out of mom, and although I've been a "daddy's girl" my whole life, I always carry the memory of his selfish weakness with me.
Don't get too insistent or crazy when your daughter calls you "Mama Rosalind." Just hold her and tell her that you're just mom, and that you'll always be her mother, no matter what. She's going to figure it all out for herself, and the only thing you should do is let her know, while she does (or until big brother fills her in), that you are always going to love her.
Peace out.