As Brady later said, he only said that in the heat of anger, and did not mean it. He told Theresa that. She kept asking him over and over to come be with her, in fact, she even said to him, make love to me just one last time, here and now (in the Kmansion). She was really ticked she could not get him back. Don't forget that Theresa very willingly left Chloe behind, that in 18 mos. she could never come up with any sort of evidence to be used against Mateo, and then she very willingly called Mateo to warn him John & Paul were coming.
And yes, the actress is gone, she was only visiting for a short stint, which is why her storyline was ever worse than everyone thought.
Theresa's visit to Salem is no different than the parade of "guests" we have had, and will continue to have over the next several months. Short stays.......Hi, goodbye.