All this advice has been super eye-opening and helpful. I am heartbroken, and wondering if I am just prolonging more heartache by staying in this while he figures out his shit. Everything felt so real when this relationship started, I trusted his words and his actions, which has led me to a super-dramatic situation. The wife has sent me messages.
You don't need to respond to her messages. (You can even block her if she's being intrusive or upsetting.) The relationship you have is with the guy, not with her.
... stating he has fallen in love with me, and this is the last straw their marriage can handle, while also begging him to not leave.
She can feel things and want things. That's none of your business. You won't stay with this guy (wait for him, whatever) to please her, or even to please him.
You'll stay because you feel, rationally, realistically, not just romantically, hopefully, impulsively, that you have potential for a happy and healthy relationship with the guy (whether he is legally separated, or stable in an open relationship, which, right now, he isn't). You were led down a garden path. You thought things were great. They are not.
I am in the middle, holding all he has shared with me regarding their marriage, hoping he makes a healthy decision for himself, which would be to leave his wife and follow his heart.
You're not in the middle! You're just an innocent bystander. HE is in the middle. He's the hinge. He has to make some hard choices. The choices are pretty clear. The decision is hard, but it's for him to make. Step back, step out of it. You don't have to convince HIM. He has to leave for himself, not stay for her, or leave for you. It's his life.
If he's telling you one thing, and letting her believe something else, that says something about his character, and it's not good.
Will he or won't he? Can I stick around to find out?
Look back to what I said about being in a similar place. I didn't "stick around." I went about my life. I didn't wait for some dude I barely knew to affect my life, or my future, or my dating, like this. I don't care for wishy-washy people. I want dependable people in my life.
I know you're probably in shock at how this situation has deteriorated. It is very upsetting to go from Cloud 9 of NRE to suddenly having this mess instead. Be good to yourself. Try to take breaks, get off your phone, go outside, see friends, etc. Give your brain a break.