Rebecca's original plan was to expose Ethan and Theresa in front of everyone the moment she found the letter, which originally Gwen went along with, until she heard Ethan tell Theresa that anyone who tries to come between him will be out of his life forever. At that point, Gwen told her mother not to do it. She wanted to expose Theresa, but not incur Ethan's ire. So Rebecca had to back-peddle...she had gathered everyone together, so she had to say SOMEthing...that's when she came up with the engagement party idea. It wasn't a plot, but a spur of the moment cover-up.
