It seems to me you're in a pretty hard situation.
He seems to get the "good parts", and when you need help dealing with the situation, she just abandons you? I can understand it would be hard for her too, but one way for you to get better over it is for her to make you feel important, and, well, while there are people who want all the attention, you don't seem to be that way at all.
It's important that she recognises her responsibility in breaking your boundaries and cheating on you. When you fully accept poly, if you do, that won't mean what happened before wasn't cheating: cheating means breaking the rules, and she certainly did, and seems not to have cared much about it, and blamed it on you instead. She needs to take responsibility if she wants it to get better.
The fact that you can't meet him worries me. It would be much better for you to have some contact at least. What if something happened to her and you needed to get together to work through it? What if you wanted to plan a surprise for her together?
Separating the two and leaving a "double life", I could understand if it was really the same, if she lived half the time with one of you, half the time with the other. But at times it sounds like she's acting like a teenager, being out all day or night and expecting you to take care of the house in the meantime.
And the fact he doesn't want to meet you... I'm worried he's either trying to recreate a cheating situation, because he's more comfortable with that, or that he's a cowboy. Either way, I feel he needs to really acknowledge the situation for what it is: a relationship involving all three of you.
Dealing with jealousy is always tricky. But right now, I can't really blame you, it seems like she decides and you just have the right to take it with a smile.
Poly relationships require time, patience and communication, especially poly/mono ones. It sucks when you're the one who has to slow down because a partner requires you to, but if you value them and they're not being incredibly unreasonable (and you don't sound so) I feel you need to take the time to show them everything is fine, until they're comfortable enough that you can proceed.
She's going too fast, without taking you into account. By doing that, she risks reaching your breaking point, when you won't be able to take it anymore. If she slows it down, she's likely to be able to keep both relationships as long as she takes everyone's feelings into account.
Being the hinge means you get two people (or more) who care for you, but also that you need to take care of both (or all) of them. It's not all fun and games, there is a lot of responsibility involved in it.