So why should this not apply to Milah too? I would argue that in the context of the show, it’s likelier that that her poor choices stem from unpleasant experiences than not. Besides, we do have evidence that she probably did hard labour (hauling wood in ‘Devil’s Due), she was depressed (she drank, she left her family!), and we know she thought things would be better if they moved, but Rumple didn’t want to. It’s not about probability, it’s about giving Milah the benefit of the doubt.
I think I agree with most of your points. The problem isn’t so much Milah — it’s the overall grammar of the show that frames Milah’s actions, and which tends to be pretty black and white. On OUAT, you can be a terrible partner/significant other without being narratively framed as a completely hopeless villain (case in point, Arthur), but you can’t be a bad parent and avoid being villainized. In fact, being a bad parent is the one reliable symptom of villainy on OUAT. Similarly, villains are made sympathetic via their “correct(ed)” attitude to motherhood/fatherhood (hence Rumple, Zelena, Regina, Maleficient, and eventually Cora all have their sympathetic side revolve around their relationship to their children, and their redemption, when applicable, is framed around that too). So based on the way the show ties moral worth and parenthood, Milah actually is in the same league as Pan, the BF, and whatever other unrepentantly terrible parents we’ve seen. So it’s not her relationship with Rumple that makes her unsympathetic. It’s that the show systematically equates ambivalence, rejection, or improper interpretation of parenthood (see Cora & BF) with moral bankruptcy. I think we can discuss whether that’s a problematic message or not, but I think that’s what’s behind the audience’s dislike of Milah.