Are you clearing your own internet searching history- using a password protected account? You need to stay one step ahead of him to avoid him finding out what you know/tracking down your Mumsnet posts. Off the beaten track might be a good place to post in the future so that your posts don't appear in active conversations...
I agree that it is time to consult a very well regarded solicitor.
It is possible that you are blocked from that particular person's Facebook account!
regarding facebook - the woman in question probably has a private setting which means she cant be searched for - however, if your husband knows a mutual friend of hers then he can look at this persons friend list and gain access to her facebook page that way. just because you have opted to be non-searchable on facebook doesnt mean you cant be seen in your friends' friend list - hope that makes sense - although it is possible to hide your friend list not everyone does this. but anyhow thats the most feasible explanation of that
Going back to facebook. I have an 'exfriend' who doesnt show up on a search if you put in her name, but if you put in her email address then it does. Can you get her email address from your DH account and check that way?