I don't want to quote the whole post, but JSMa, just responding to what you wrote above: I really don't see this and I think focusing on the idea that the mom is instilling these feelings in DSD is actually something that keeps you away from focusing on what DSD is actually feeling.
For ex., clearly the mom is not totally invested in keeping the idea of the intact nuclear family together because she herself has also built a new family with her partner and daughter. DSD seems relatively well adjusted to that situation from what you describe. I think it is healthy that she keeps pictures of her and your dh and dsd - because, in their own way, they ARE still a family. A different kind of family - one where the mom and dad are not together, but still a family in the sense that they share care of your dsd and both love her. I assume that this is the message she is trying to convey. I know my photo albums, to the extent they exist/are organized, have pictures of my ex, my daughter and I - I also have pictures of my current partner, my daughter, my baby and I.
In terms of the memories, of course dsd does not have memories from 2 (though some pple claim they really do). But she may have "made up memories" as stories she tells herself about what her life might have been like. They might be a fantasy that she is trying to work through. They might be her way of expressing a sadness for something she wishes existed - perhaps because she's getting older and realizing that not everyone splits their time between mom and dad but some kids have a mom and dad they see every day.
She can have all these feelings without her mom promoting an unhealthy attachment to the past. I would be more inclined to buy that assessment if the mom herself seemed to live in the past, seemed to want more contact and involvement with her ex and had not moved on in her life and relationships. But that's clearly not the case. I would imagine you'd be more upset if she had taken all pictures of your dh out of her house and "moved on" with just pics of her, the boyfriend and dsd.
I think the fact that you think it's unhealthy for the mom to talk to dsd about her past family, to keep those memories for her, etc, is a way of saying that you feel dsd's feelings are unhealthy or somehow wrong. I don't know how or if that gets communicated, but that can be really difficult for a child. She's going through a LOT of change. I think you guys really need to face her feelings as HER feelings and listen and try to understand and empathize.
One thing I really liked about the book Between Parent and Child (which How to Talk... is based on) is that he tells parents to try not to focus on "fixing" things or talking about how things could be better, but instead on mirroring feelings. In a lot of your stories I hear you say "you must be sad, miss mom" etc but then try to re-direct or focus on some project. I would instead try to really mirror when she says she's sad and stay there for as long as she needs. Maybe err on the side of talking about how sad she is a lot to the point that it seems like she's dwelling on it. But for her it might be empowering and comforting - she may feel heard and supported and like her feelings are totally normal. Just a quick example of this: my daughter (10) was going to Egypt with her dad to see his family there for 3 1/2 weeks (a really long time away from me - she has a hard time doing more than 2 days. Two days before the trip she freaked out, insisted she wasn't going, was crying, etc. Instead of trying to talk about things we could do while she was there so she wouldn't miss me (like Skype) or all the fun things she'd do there or how much it means to her dad, we spent an entire day just listening to her feelings over and over no matter how "stuck" she seemed. At the end of the day, she went to bed still saying she wasn't going. I was terrified of putting her on the plane two days later. The next day she woke up cheerful and ready to go and focused on the future - and she ended up having a great time and not having a meltdown about missing me a single time she was there. I don't promise that kind of immediate result but it really validated for me the approach.
One quick mental shift idea. You end a lot of posts asking "how can I make things better for this poor little girl?" or something like that, which is really great that you care and feel that way. But maybe the goal would be to ask "how can I better understand what she is feeling?" and let go of trying to "make it better". Kids have enormous internal resources to fix things themselves IF they are helped to identify, express and understand their own feelings.
I think your dsd's feelings are really painful for you and especially your dh. You work SO hard to make a nice home for her and for her to be happy there. You had/have a lot invested in creating a new and happy family with your new daughter as well. There's a lot at stake there and I would definitely be in counseling (for you and for dh) to talk about the feelings all this raises for you. But I would try to really also understand how that is working into your approach to your dsd and try to consciously step back from your own hopes and expectations and instead focus in on what hers are.