You're right, I'm certainly not an expert in psychiatric disorders.
However, "attachment theory" isn't directly to do with psychiatric disorders. It's about the possible effect on a child of being (temporarily or permanently) deprived of a primary carer when young. In SOME cases, this may lead to mental health issues later. Trainee teachers and psychology students (and I've been both) are required to study it.
It's a theory, rather than a certainty (google John Bowlby, if you like, and you'll find out about it), but it makes some sense to me.
You’re right, “attachment theory” isn’t directly to do with psychiatric disorders. And when June had her breakdown Nevill was the primary carer, including the nanny. So Sheila wasn’t actually without a primary carer: she wasn’t in a children’s home being cared for by different people each day.
I too have studied psychology, and what you need to remember is that each individual, regardless of age, is affected differently by family upsets/traumas. You also need to remember that some families have a parent who is absent for weeks on end: businessmen, tycoons, airline pilots, off-shore engineers etc...and they don’t all develop MHP. What DOES affect a child is the behaviour of the parent when they’re home...
Jeremy certainly seemed to be affected by whatever went on at home. But his problems were vastly different to Sheila’s. Unlike Sheila who was naturally sweet, placid, and caring — Jeremy was spiteful, arrogant, nasty and cruel. So it stands to reason that he had deep hatred inside, so much so, that he was able to kill everyone.