I'm sorry to hear of your troubles, OP, you must be really worried. I truly hope the medical diagnosis is either negative, or if positive, is easily treatable.
With regard to your ex it's not your fault your marriage fell apart. You didn't hold a gun to his head and tell him to fuck someone else did you? No. Well then, it's his fault, not yours. He's just trying to transfer his guilt onto you - my ex did the same. However you are not responsible for someone else's actions, only your own. The good news is he is now her problem, not yours.
The bullying of your DD by the OW's DD is unacceptable and tbh if it's resulted in your DD needs counselling then I'd be seeking legal advice about where you stand in stopping contact until the bullying issue is sorted in full. Bullying is about control - the OW's DD clearly thinks she has control over your DD, as does the OW and your ex because they're not stopping it from happening, and that's wrong. As your DD is 8, then ask her if she wants to still see him in this situation and if not, tell him this and seek legal advice. You may be able to get some kind of legal paper that says your ex can only see her away from the OW and her DD due to the bullying - I'm sure no judge would be willing to let this continue.
I had an ex who cheated on me when I was pg and I hoped karma would 'get' him. Unfortunately there's no such thing as karma. The only thing you can do is look after yourself and your DD as best you can (as you have been doing). The best revenge is living life well, so that if you see them in the street/they come crawling back/they ask a mutual friend about you, it's like a big fat 'FUCK YOU' to the ex. As someone else said upthread, few OW relationships last because they are built on a foundation of lies and deceit, so how can there ever be any trust? When the OW becomes the partner she creates a vacancy, which one day your ex will most likely fill. As I said earlier, this is now the OW's problem, not yours.
It's nearly the end of 2012 - I'm sure 2013 will be a million times better for you. Chin up, and keep us updated with the medical results.