I don't understand the omega. I was sure he wanted to meet the alpha to put an end to their relationship.
Does he really want to marry the alpha ? 
I know it's for show and he wants to use him to dceive his mother but it will hurt our beta and the alpha is still ok to marry him after being abandoned for 3 years 
He's not going to marry the Alpha. He is just doing this so that the alpha will think he's going to marry the omega of his dreams and finally let the beta go. If he thinks "Well, I guess I'm going to marry my omega" then there is a big chance he will let the beta leave. Once the beta is out of his reach the omega will most likely dump that alpha at the altar or call of the marriage at the last minute in a way that screws the alpha as much as possible.
He wants to the alpha to make the beta completely hate him (mission accomplished). The minute the beta is freed he is never going back.