Were does it say that Elvis promised to "marry" Priscilla eventually. I have not read that. I believe he promised to take good care of her and treat her well and make sure she finish her education.
Also, Elvis talked her father into letting her come only after, Priscilla begged Elvis to do so. And after Anita Wood left.
I believe Elvis was infatuated with her,,,just like he was with soo many others. And he wanted a women at home. (Anita was living at Graceland) So he replaced Anita with Priscilla. But it was for his comfort and entertainment in many ways.
His problem is that he kept her there too long. Untill he married her she was not going to leave, even though he was by all accounts long bored with her. She knew he ran around on her and still wouldn't leave. Funny that all the women say they knew he had others and accepted that was Elvis, but all of them left him and did not marry him.
She wanted to be Mrs. Elvis Presley weather she loved him or not by the time they married. And I don't think they did much. It was a marriage of convience mostly. That is why it was never stable.