I think Forest said that if Nicola had been there instead of Lawrie that would have been 'one more brave, resourceful child helping to cope' along with Peter and Patrick, so Lawrie was required to introduce some randomness and panicking into the plot (which she does very effectively).

Peter's non-officerlike qualities come out more with his family, maybe (thinking about him bullying Nick in the kitchen in ?Peter's Room), but his poor judgement of character would be a serious flaw. Is he improving with age, though? Clearly Selby is all right, and he's not the only one to dislike Edwin. And I agree with you that he copes well in Run Away Home.