It is essentially the David O. Russel subverted version of Elizabethtown or Garden State. Instead of quirky Midwest/New Jersey family/friends, it's quirky Boston family and friends. Instead of a over-the-moon happy girl trying to lift the spirits of a down guy, we have a solemn, moody one trying to lift the spirits of a bipolar guy. Dunst, Portman, and Lawrence are all the main reasons for the male protagonist's improvement as a person. She likes dancing, yes, but it's a "in" to a guy. I would say from the get-go that she was never interested in actually helping Pat (she already shows concern and affection for him early on AND she already knows where he runs... this was planned WITH the family which goes to my other point about saving him - his Mom and Dad don't want him to get back with his wife, they obviously want her to get with him... all of this took place BEFORE she asked him to help her with the dance, dude). In that very specific scene in the kitchen, she explains to his parents about her troubles with "helping him". Russel is fascinated with men with mental problems as shown in I Heart Huckabees and to a somewhat lesser extent, The Fighter. But this one was so bland because of how planned out the entire thing was. Not to mention that the father's subplot is never really explored (his restaurant idea seems like a faint pipe dream that never gets the supposed weight we are supposed to feel) but that's another story.
Um, you dont seem to be grasping actual details of these movies you're watching, so no wonder I find your criticisms perplexing.
DeNiro's restaurant - which isn't a subplot to explore anyway - is explained by the brother as a means of laundering his bookmaking, so there is no "weight" to "feel" plus it gets its mention in the little coda at the end
Her collaboration is firstly with the mother and only after meeting Pat when fixed up by her sister: her sister is pushing her onto Pat before she is calling Pat's mom. And at the fix-up dinner, she bristles in embarrassment when the dancing is brought up by her sister. Only after her confrontation w/ Pat's dad (again: her own agency) does he come around to liking her, having been verbally against their spending time together beforehand. So under your rubric, Julia Stiles is more a manic pixie dreamgirl than Jennifer Lawrence, and the mom is the most manic pixie dreamgirl of them all. So don't worry, you're cool bro, there ARE manic pixie dreamgirls in this movie so you CAN prove you're too cool for that trope and dismiss this particular film based on your distancing maneuver. Bravo!
Once again: her interest in dance is not an "in" to a guy because, like I already said, she was into it when she already had a husband. And if she doesn't want to talk about the dance at her fix-up dinner and doesn't angle him with it at their diner date and we watch as she shifts her feelings for Pat when he stymies the office fling and that immediately motivates her to ask him to do the dance (she had to come around to trusting him w/ something so important to her, something more important than he is to her at that point), then how do you still see her interest in dance as not her character's personal interest but instead what you claim is a contrivance for her lifting his spirits?
So she's "solemn" she's "moody" she's selfishly "never interested in helping" - how is any of that manic or pixie or dreamgirl?
You're sitting on this buzzword that you've apparently read is a bad, cheap device in movies like this (and you don't even seem to understand?) and you can't see beyond it when there's an actual developed relationship in a movie.