Good job, you handled it very well.
She cried and cried and confessed that she and Mark do have feelings for each other but she is adamant that it's progressed to nothing physical.
So she's admitted that she's been lying to you for months. Your previous fights, accusations etc. it was all her fault. She manipulated you to hide her A. It's not just her cheating, but these attitudes also show her personality.
By the way, now that she confessed to EA and had so many opportunities, you probably know by now that their A was fully physical.
While it was a stupid lie that they smoke weed and just chat until late as friends, it's a far more stupid lie to believe that they're not physically involved as EA partners now. The WS mind never changes.
saying that I'd "already made up my mind without her".
You would have included her if you could find her. Meanwhile, she was having a good days with Mark. In fact, if she was there for you, you wouldn't need to do anything without her anyway.
Stupid cheater mind still.
Then she started begging, crying, saying that it seems like a rash, drastic decision to divorce and that we should try separating first.
After all that crying and begging, at least here, she would be expected to say, albeit falsely, that she loved you and wanted to stay married to you or something. If you don't feel these things, why are you crying and begging? But no, she just wants time to put the ducks in a row.
If things go well with Mark, she'll come over and say, "Okay, honey, let's get a divorce, why didn't we do this before, why did we even get married?"
If things don't go well with Mark, she'll come back this time and say "I realized that all I wanted was you, even when I was with Mark, a part of me always loved you." Instead of "Mark didn't want a serious relationship with a cheating, lying woman, he just wanted to fuck me" of course.
I booked a flight to go stay with my family this weekend.
Tell your family what happened and that you're getting divorced. Their support is important at this time. You should also call your WW's parents and tell them what happened and that you're getting divorced.
Btw, if you're still in doubt as to whether their A is physical, or if you need tangible proof, you can hire a PI while you're with your family.