Hey SunnyD, I don't feel qualified to offer advice on how to respond to H's last email, but really wanted to say hi because I'm finding myself in such a similar situation to yours: floored by a bomb out of nowhere and completely clueless as to where it came from: PA? EA? MLC?
As do you, I doubt the PA, but EA and MLC both seem like possibilities, especially because of H's Facebook obsession. And the complete blame that he's heaped on me sounds so familiar too. On top of that, the reasons I'm to blame kept changing ... there's no one good answer or explanation. It is so frustrating but, like you, I have no way of looking deeper: no access to FB, Blackberry or co-workers (H moved out months ago, leaving the kids alone with me).
Sorry, really just venting a bit and commiserating with someone who appears to be in a very similar situation.