Ahh, it all makes sense with your updates. I think you know that this is nothing to do with snobbery on her part. That’s just an easier story for you both to tell yourselves, to place the blame on her and her character flaws, rather than face up to your own parts in this situation. I think your DH is far more responsible for this than you, but you both played your parts.
i cannot believe you have not worked out the source of the problem- are you being disingenuous? It’s just that you’ve just told us clearly why she is not close to her dad (and by extension, you), so it’s hard to understand how you can’t see it?
I think it is clear that after only a short time of entering a relationship with you, her father began to very obviously prioritise you/spending time with you over being there for her. He stopped being involved in important aspects of her life. All this happened not that long, in the grand scheme of things, after losing her mother and all the turmoil that must have brought. You admit that you both moved too quickly with introductions and tried to force a happy family before she was ready. That was quite possibly very damaging to her emotionally, and almost certainly to her relationships with both her father and you. Have either of you ever acknowledged that you recognise that these things weren’t fair to her?
I’m sure there were a number of reasons for her going to Canada to study, but I’d bet one of them was to try and put space between her and a situation in which she was unhappy and to distance herself from a father who hurt her. It might have been easier to be far away, so she couldn’t have the fact that she wasn’t a priority rubbed in her face, for instance. Then, once she had left he moved in with you- so now she effectively had no home to come back to- why would she bother? I’m not saying you would not have allowed her to stay, or that you would not have tried to be welcoming, but I think it’s understandable that she would not have felt comfortable moving into the home of the woman who had become her father’s priority when she still needed his support. And possibly, whom she felt some animosity towards because she was forced into having a relationship with and, moreover, playing “happy families” with, long before she was ready.
I think she is trying to maintain a relationship with her father, but doing so on her terms- perhaps she is trying to protect herself from being hurt by controlling what she does/where she stays/who she sees. It’s hard to say how her paternal grandparents fit in there- maybe she fees they haven’t been supportive of her, but that her maternal grandparents have been, therefor she feels comfortable visiting the latter? In any case, I think it’s clear she doesn’t really want a relationship with you or your son/his family- maybe she feels your DH also prioritised/continues to prioritise your son/your grandchildren over her too?
I don’t understand why your DH cannot visit her without you? That way you could look after your grandchildren as planned, and he could still see his daughter and his grandchildren. My guess is that he won’t do that as he can’t be bothered to make the effort, or because you don’t want him to do that, for some reason.