First off: obligatory "It was a different time", except I do want to emphasize it, because it's not only to contextualize some of Mimi's behaviour that a lot of people might classify as emotional abuse nowadays, but because it affects literally all of this. Social mores dictated the actions not just of Mimi but of all the Stanley sisters, who in my view all enabled the situation to get as far as it did. And, most pertinently, I don't think people at the time really understood how damaging it is to all parties to separate a child from their parents (and tbh, especially their mother). My main takeaway from the whole situation, as well as what I've learned about "difficult homes" since, is that taking John away from Julia should have been an absolute last resort, unless there was extremely concrete evidence that Julia was being highly abusive towards him.
(and I do mean highly abusive. It, for instance, would not have done Paul and Mike any good to be separated from their father, even if Jim put them through hell at times [though of course, the extent of the abuse they suffered is debatable]. These situations are terrible all-around, but from what I can tell – and what I think John's story shows – breaking apart families is very rarely the best course of action.)
"But Julia was making little John share a bed with her and Bobby Dykins!" – okay, I see how this is potentially a problem, though it's from what I can tell entirely (implied? I don't know that she went so far as to say it out loud) conjecture on Mimi's part that John was made to witness adults having sex. In my opinion, the first course of action here should have been to help Julia out by acquiring a separate bed for John.
I recognize that Julia was probably often irresponsible and, from what I've read about her, I suspect she at times struggled with parenting in way similar to how John would later on – not due to a lack of love, per se, more due to a helplessness in the face of the true burden of taking care of someone and a difficulty comprehending the seriousness and scope of child-rearing. That being said, it's hard to fully blame her when she had her toddler taken away – because of her own sister, as you mention – and had to give up her second baby for adoption. That's twice-over the worst trauma a mother could endure* so it seems natural to me that she would have attachment issues with John later on. Again, helping Julia with her parenting instead of denying her the role of mother seems to me the best course of action here. (Oh look, it's my nuclear family take rearing its head again.)
*some people happily give up babies – I don't have much reason to believe Julia was one of them.
I also think Mimi thought she was doing the right thing and her decision to take John in admittedly made more sense when Uncle George was still alive. This is where the "It was a different time" comes back in, because it would be unfair of me to expect her to understand all the complexity I've been describing. I don't know that I believe Julia Baird's claim that Mimi instigated the entire situation so she could have John for herself – but there is perhaps a middle ground here, where Mimi loved her nephew a lot, felt no one was taking care of him the way she believed he needed, and let that blind her to the damage she was going to inflict by interfering the way she did.
It's difficult to fully assess Mimi's impact on John, because it seems the abandonment issues his early childhood instilled in him made him particularly loyal to her. Mimi was, if nothing else and above all, steadfast. (You can draw a parallel here to Yoko, though that's another discussion.) You aren't gonna find a ton of statements from him condemning her, and I definitely don't want to discount the genuine love they both had for each other. I appreciate that she didn't have an easy task, bringing him up, especially after being widowed, and I appreciate that she came through for him in significant ways.
"You wonder why John didn't have the strength to just say 'Look Mimi, I'm moving in with Mum.'"
JULIA BAIRD: "Hm. Well, I remember the– a particular row in the kitchen, where he was going back [to Mendips] after the school holidays and he was crying. And my father [Bobby Dykins] said 'That's it. That's it. I've had enough. You stay here. I'll go and see Mimi. I will sort this out once and for all.' And John said 'Nononono, nonono. I've got to go. Don't tell Mimi.' That was another refrain [?]. 'My dream's out.' and 'Don't tell Mimi. Don't tell Mimi, don't tell Mimi.' She was fearsome, I can tell you. Fearsome."
SOURCE: I am the Eggpod, Episode 100, 00:49:49
What I hate about the above quote the most is how often John, even as a teen, is conceptualized as this free-spirited rebel*; but looking at this, it hits extra hard that he was a child who had virtually no power in this situation the adults around him put him in.
*and writers like Mark Lewisohn are definitely not off the hook here; one of the things I find the most questionable about him as a biographer is that, despite his painfully obvious bias towards John, he still fails to contextualize huge chunks of John's childhood, preferring to defend Mimi at all cost.
Now, it does seem like there's some he-said/she-said going on regarding how much Bobby Dykins and John got along. I don't think John spoke super highly of him, but I also don't recall him expressing any larger issues with his pseudo step-father. Yes, Julia Baird is probably biased towards her own parents, but here's the thing: don't most of our pro-Mimi accounts come from her? Is John an unbiased source on the aunt who raised him?
So why should Julia Baird – whom I consider to be one of the most empathetic characters involved in Beatles history; rarely have I seen someone so interested in truly understanding their own family history, considering the cultural context, tying in her experience as a teacher in special needs and talking to multiple people to gain a fuller picture, even when a lot of it must've been incredibly painful for her – be seen as a less trustworthy source than Mimi – who we know has a history of editing stories to suit her:
I [Hunter Davies, the Beatles' official biographer] sent [the manuscript of the biography] to Mimi and she had hysterics. The chapter about his childhood came back with almost every paragraph heavily crossed out or amended. In the margins she had written beside John's own quotes such things as 'Rubbish', 'Never!'
She denied so many of John's own memories of his childhood, especially if they contradicted her memories of the same people or events. She was against his use of bad language, as she maintained John had never sworn when he was little, and didn't want stories about him stealing.
SOURCE: The Beatles by Hunter Davies, 2009 Introduction
The only thing Mimi has "going for her" as a source is that she was an adult and not a child when these events transpired. I think that's fair enough, and I do believe there are details to this story Julia Baird and we the public will never know, which made Mimi's decisions seem more justified to her and to her family. But that doesn't stop me from largely condemning most of the decisions made, even though I suspect Julia did at least on the surface agree with them.