How Being a Good Son Saved Edmund Bertram.

By the end of the book, Edmund Bertram is set on proposing Mary Crawford, which would have probably led to a very unhappy marriage, but this doesn't happen as certain other events regarding Henry Crawford and Maria happen beforehand, which help him see why he should not marry Miss Crawford. He had initially planned to go to London to propose to her earlier, which would have been before the adultery, and led to him marrying her. Edmund only delayed his plans to go to London and propose as the other young people (the Prices and the Crawfords) were leaving at the same time and he didn't want his parents to be alone. This all means that it was indirectly due to his decision to help his parents comfort that he was saved from an unhappy marriage with Miss Crawford.

Edit: Upon rereading, it turns out that I may have been misremembering as the Henry-Maria scandal doesn't happen on the visit to London which Edmund delayed, but rather later. This visit to London concludes before the scandal and Edmund returns to Mansfield where he writes Fanny a letter where he confides in her that meant to propose but that he was kept from doing so because when he met her in London the qualities in her her that gave him scruples had returned.