Maria breaking an engagement would be bad in itself, and may damage her marriage prospects a little. However, Sir Thomas was ready to go through with it for Maria's sake. Perhaps the fact that the engagement was contracted in his absence may lend some palliation in the eyes of the world.

However, breaking an engagement only to contract it with another would be a big no-no. Sir Thomas does not know Henry Crawford. And everyone for miles around Mansfield Park know about the MP Theatricals thanks to Tom. It would be easy to guess what had happened, even if Mr. Rushworth kept silent on that issue.