Just using your post to jump off. I know that NM had confessed to Becky's murder at this point, and was probably horrified at what he had done (giving him the benefit of the doubt here), but I though it was telling that he was the one who appeared to be distraught, was crying and did seem to be upset by what had happened, in his interviews.
Whereas SH is very calm in hers, right from the start. I know she didn't like Becky, but I can get upset and tearful when hearing sad news about people dying, I've cried when talking to a friend whose son I had never met committed suicide, so I do find SH to be rather calm and collected when she has just discovered that her long-term partner's step-sister is dead.... that a teenager she had known for around 6 years has been dismembered.
Of course people are right to point out that she was only 14 when they met, legally still a child - but what type of man in his twenties is interested in a girl so young? For a start, a very immature one! Girls are usually more mature for their age, and if he was particularly immature (which I don't think is impossible to rule out) then the gap in maturity levels could be a lot less than it was in actual years. So, I don't think that we should automatically assume that SH was moulded by NM.
I think her composure and measured answers are what makes me find it hard to accept the submissive, controlled role in the relationship. However, like I've said before, we are only given snippets of evidence to make judgements from, so its all supposition and opinion and nothing else.