I've been thinking about you all evening, BF. I was a bit scared to post because of your evident (and justified) fear of your predicament, so I was glad to see AF here. I trust her, no probs 
I hope you will start another thread. In the meantime, here's a (slightly) shorter version of what I'd like to have discussed if you were in my house tonight!
I hear your insecurity, anxiety and desperate desire to be 'wrong'. Just thinking about it brings back the awful, howling vortex of doubt, which I've felt too often. I remember it while pregnant, too. It seemed unbearable.
So I completely understand your need for it all to go away; for the two-and-a-half of you to get back to normal.
But you know that 'normal' has gone. You didn't break it, he did.
Hardest thing, then: Face this simple but important fact.
People do not contact escorts on a whim. Either they habitually contact escorts or have been thinking about it for some time, then decided to go ahead.
They might, conceivably, do it while off their face but, actually, a man with no tendencies that way wouldn't. In any case, you've covered that - he was calm and sober.
Unless you have a steady history of paranoid mistrust or possessiveness, possibly even a psych diagnosis, you can trust your instincts. Feeling "irrationally" mistrustful IS rational! It's not magic, it's the outcome of real observations that you have stored unconsciously.
It was rational to snoop his email, and you knew it or you wouldn't have done it.
He's gone behind your back. He's insulted your relationship. He's changed your future. He did those things.
What you can do now is take back your share of the power. Don't commit yourself and your baby to a life of doubts.
I hope you'll start a thread to get Mumsnet's finest working on that with you!