I guess its impossible to say whats brought it on without knowing loads more (and even then, it might not be possible). Without sounding like a cliche, it does sound like you are having a hard time at the moment, and children do pick up on stuff, whatever we do to try and shield them from it. 'Acting normal' for anything other than a short period of time must be tiring and anxiety provoking for you in itself. Interesting that her brother has Aspergers and i guess its possible she has observed some similar traits in him (hard to tell without knowing more).
Maybe she has picked up on some of whats going on and some of her behaviours help her to cope? Maybe for now that's not such a bad thing?? I guess it just reinforces what i said earlier about focusing on helping her to develop coping strategies rather than making a big deal of the obsessive stuff.
Try not to get too focused on the individual behaviours and the fact that they don't make sense to you. I suspect for her at the moment its working and helping her cope. If she is picking up on stuff and finding it hard, what would you prefer her to do instead? And how can you help her work towards this? Its also worth thinking about what kind of messages she is getting about emotions from the people around her. Children primarily learn about how to handle their own emotions through watching others. When people around them pretend to be feeling something they are not (putting on a good front, so to speak), it means they don't have the opportunity to learn about a full range of emotions and how to deal with them in day to day life.
In the medium to long term, your situation sounds far from ideal for all of you and i guess ultimately for everyone's sake you need to move forward. Is it really impossible for you and H to live separately? I'm no expert on PND but it seems much more likely that you will get better when you are living in a happy environment without the added stress of trying to act 'normal' for the sake of the children. Have you had another child since DD?
Hope that makes sense. Please be kind to yourself - its not about blame. Life is full of ups and downs for all of us. Ultimately its not possible for any of us to totally shield our children from all of life's tricky situations, but it is within our grasp to help them to develop ways to deal with them.