That thread is worth reading. It's by just a case of you found out you are your boyfriends OW and should you tell her.
That's why you got such differing opinions. My opinion is still the same. You know a lot about her to say you knew nothing about her, including about her relationship with his kids.
You were not in a relationship with this man and was sleeping with him when you knew he had other girlfriends. That didn't bother you.
Their relationship is complicated and it appears she had an affair with this man, has left her husband and now is with him. He has now blocked all contact with you.
This seems to have become an issue simply because he has blocked you and cut contact.
You need to move in and stop putting so much energy into their relationship.