I am going through very similar. I imagine that the issue with his letter writing, cestlavie is that the letters have to be responded to. My ex's solicitors write bullying and inaccurate letters, which must be replied to. As each party pays their own costs, then this is incredibly financially draining (as well as emotionally draining). Therefore the intention is to make my money run out so that I can't fight the court case, as he has access to the family/business funds.
The only advice that I can give is to try to remain calm, don't rise to issues that don't require a response and stay focused on those that do. Do what you are supposed to do and don't do anything that you're not. Even though it's very emotive, try not to get embroiled in tit for tat as it doesn't actually progress the situation, and runs up extra bills.
It's very difficult to keep it totally from the children as it's an incredibly stressful situation, but it's the only thing to do-they need to feel as secure as possible. I just try to remember that one day, all this shall pass and I can then get on with enjoying the rest of my life (much poorer, admittedly!!).
Have you got people that you can off load on in RL? Have you thought about contacting children's services for possible counselling for the children? Are you happy that you're getting good advice from your solicitor?
Sorry that you're going through this and I hope things get better for you.