I think if Mrs Clavering is being characterised for being "malicious and unhelpful" for saying the little she does say, it's worth bearing in mind what she could if fact say if she weren't being restrained by good manners.

At the moment we see this relationship it's a train-wreck in progress (which may explain why there are so many train-crash metaphors flying about the place, come to think of it) and if it does end up OK it'll be more by good luck than good judgement, the latter being remarkably absent from any of the principals.