Hmmm, this is an interesting question, but I think you’ve got a couple of things tangled together.
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Normalcy is definitely important for both John and Mary. I don’t necessarily think that it plays a primary role in their relationship with one another, but I think it’s got vast and deep effects in other parts of their lives.
Your questions are predicated on a foundational point that John and Mary established their relationship based primarily on a desire to maintain an appearance of normalcy. I disagree with this. My own belief is that their relationship is established primarily on a personal attraction and love for one another.
Mary said she would kill to keep John by her side—not in order to protect her normalcy, but because she loves John and wants to keep him. At the time she told Sherlock this, she believed that this meant protecting her secret from him, because she thought John would leave her if he knew.
(One might well ask what sort of love we’re talking about, when someone declares their willingness to commit murder in order to preserve your affections for them. Silviatietjens is quite right about Mary’s love for John being selfish, but then that’s not surprising coming from the sort of woman who would work as a contract killer.)
John likewise chose Mary because he was attracted to her and loved her. That this is his primary motivation can be seen in the fact that he takes her back even after he finds out the truth (about her, and about himself).
However, is normalcy tied into his relationship with Mary? Absolutely. We see that too, in that confrontation at Baker Street. John does clearly equate Mary with ‘normal,’ and further this is clearly a comfort to him, because when he’s forced to question that connection, it rattles him profoundly.
When he discovered that he was falling for her, I think that it was a bonus for him that he seemed to be falling for a ‘normal’ girl, and that he took it as evidence that he was a ‘normal’ man. (What ‘normal’ ever really consists of is, of course, always a question.)
Now what’s REALLY interesting is why this would be important to John (we can see why not rocking the boat would be beneficial to Mary).
We have repeated insinuations that John’s emotional distress over losing Sherlock was profound and long-lasting. From the way Mary reacts to Sherlock’s reappearance (“Do you know what you’ve done?!”) and a couple of other hints we get, I think we could go so far as to guess that Sherlock’s loss was earth-shattering for John. And before he met Sherlock, he was struggling then too (John himself phrases this in terms as drastic as “Sherlock saved me”). We have no idea what his life before that might have been like, but as Sherlock says, he was a doctor who chose to go to war. Though there are many doctors who do this for many different reasons, John’s reason is over and over presented as being connected to his attraction to danger.
As someone who has a hefty streak of non-normalcy running through him, we can presume that John has probably also felt the effects of this. He may be aware that his predilection for danger and fear is viewed askance (or even as dangerous) by society at large. A bad therapist could even have exaggerated this, if they approached it in a way that John found stigmatizing.
In any case, unlike Sherlock, John does not revel in standing out as being different. So ‘normalcy’ would be important to him in three possible ways:
1: As he tells Mrs. Hudson, he is ‘moving on.’ That he has fallen (he believes) for a nice ‘normal’ girl is something he takes as a sign of recovery (at least from his latest egregious bout of traumatic non-normalcy).
(Incidentally, I like how John refers to finding a fiancee after Sherlock’s loss as ‘moving on.’ It does imply that he views his relationship with Sherlock on a similar level, and in a similar light, as his relationship with the woman he’s getting engaged to.)
2: Being able to be normal when he wants to be means that John can still participate in mainstream society. In the parlance of social discrimination, he can ‘pass.’ While he values his relationship with Sherlock and the world Sherlock brings him into, he also doesn’t want to give up the world the rest of us live in.
3: If John has suffered at the hands of others for not fitting in, then for him normalcy may be something to be cultivated as protective coloration. To be seen as normal means not being hurt.
This: “that John’s ‘fault isn’t blame but a flaw-danger tendency PTSD” is not so much theory as what is stated baldly on the show. ^_^ It’s exactly what Sherlock tells John about himself, that he is “abnormally attracted to danger.”
The fact that John reads that as being his own fault is, I think, a miscommunication between them. Because Sherlock doesn’t CARE. No, more than that, he PREFERS John the way he is. Sherlock has established that he doesn’t blame people for not being normal; he congratulates them. So here he’s not trying to place fault, he’s trying to clarify. But John reads his phrasing the way many people would (especially if they have a previous history of abuse or blame): he focuses on the “you” and reads it as fault being placed on himself. And furthermore, he seems to internalize it.
This also fits in with the idea that normalcy is important for John, and is, for me, the strongest evidence so far that John has suffered in the past for not being ‘normal.’ He’s such a strong personality in so many ways. He doesn’t take crap when people heave it at him, not even from Sherlock. He’s confident in himself, to the point where he’s even comfortable with his decision to kill a man. And yet in this, he perceives blame being leveled on him and sucks up without question, despite the fact that it hurts him so much he physically lashes out in response. That is a behavioral pattern we see in people who have been systematically trained to accept blame—one that many readers may even know from first-hand experience.
What did that training consist of? Was it abuse? Perhaps his parents blaming him for not being able to fit in with everyone else? (If alcoholism runs in the family, I could easily see this.) Bullying from his peers? Bad experiences in therapy that left him feeling like it was his fault somehow that it wasn’t working? We don’t know. But I’d bet my bottom dollar there’s something, and that John’s craving for normalcy is more than just a desire to have people like him. Many of us are all too familiar with how much it can hurt to not fit in. Sherlock knows it well. And it looks like despite his own well-built facades, John Watson knows it too.