"I should have pointed out that although she was never actually engaged she was so disappointed at his imagined defection that it felt like she had been."

This is an excellent description of what happened. It is part of her sense of entitlement that she should marry the heir to Kellynch. I'm sure her father put this idea in her head, when she was young, as you said, and she never imagined for a minute that the young man in question might have something to say about it. The funny thing is that when he returns, instead of acting cautiously, they both take up were they left off, presuming him to be Elizabeth's personal property. So much so that they don't even see his increasing attentions to Anne as a threat. Anne after all is nobody, no one wants her...(except Captain Wentworth, Charles Musgrove, perhaps Captain Benwick, and quite probably the coveted Mr. Elliot as well...)