Even if Gitta Sereny were a better thinker, this would still be a bad book. The problem comes with her method, a shrink's method: Save everything, since it might explain something. Unedited interviews from Mary and those who knew her just dumped raw into the text, with all their um's and I'm-trying-to-remember's. That's an invitation to intellectual laziness, and there's only about 25 pages of real book here. This is indeed a wretched, fatuous book--and largely for the reasons you've laid out.
You're right about Sereny's moral vanity. The demon, of course, is Mary's mother, Betty, a sadistic prostitute with a tendency (hello, Hollywood!) towards religious fanaticism. But even after we learn of the appalling treatment to which Betty subjected Mary to please her paying clientele, it seems Betty's biggest failing is her hatred of psychiatrists. It is seen as a moral failing--specifically as a basic form of dishonesty that arises from Betty's fear of anything Mary might blurt out. Betty's hatred of Sereny's psychiatric method is thus trundled out a bit too smugly as evidence of the author's moral bona fides.
And I agree that Sereny shows a preposterous credulity towards Bell. Her indulgence of Mary's pill-popping is just the beginning. Admitting that Mary sometimes went incommunicado for weeks at a time at the height of this project, Sereny pretty much shrugs and says, "Hell, who doesn't?" Mary's love for her daughter is gushed over as if she's the only one who's ever felt such feelings. There's also the way Mary and her warped-seeming "partner" Jim rail against the corruption of "the system"--i.e., a welfare state that they bilk and a penal system that has treated Mary with exceptional indulgence. Americans will find Sereny's attack on British juvenile justice puzzling, nitpicking, and even bizarre. It sounds miles more humane than our own. True, incarceration wasn't always easy for Bell. But she did ... em ... kill a couple of toddlers in cold blood.
Sereny worst indulgence involves Mary's attempt to cash in on the murders 15 years ago by working up a book proposal. Sereny tries to explain it away by faulting Mary's manipulative "partner" at the time. There's real intellectual corruption in this. First, Sereny is trying to disguise that the working through of Mary's feelings over the killings was (as you suspect) done long ago, leaving Mary very little need to do this book, and us very little need to read it. But second, it dispenses with the idea of Mary as a morally autonomous being, without which this book has nothing meaningful to say.
It's the lack of moral autonomy in Sereny's world that makes Cries Unheard so repugnant. If the book has a point, it's that we should be more attentive to children's "cries for help." According to Sereny:
"If Mary's painful disclosures of a suffering childhood and an appallingly mismanaged adolescence in detention succeed in prompting us--whether as parents, neighbors, social workers, teachers, judges and lawyers, police, or government officials--to detect children's distress, however well hidden, we might one day be able to prevent them from offending instead of inappropriately persecuting and punishing them when they do."
That way lies Stalinism. With the hindsight that Mary Bell's grisly murders give us, we can say, sure--it would have been nice if someone had intervened in Mary's case. But applying such scrutiny preemptively would ultimately mean a regime of surveillance and Soviet-style "therapy" for any child or adolescent who's the slightest bit weird. And for Sereny, practically everything is evidence of potential criminality: a tendency to suck one's fingers (a habit, Sereny remarks, that Mary Bell shared with other child-murderers), bedwetting, pissing on the floor before running away, running away itself.
This is a recipe for a reform-school archipelago--for a society that serves the interests of psychiatry rather than vice versa.