This clue to something that may have happened before the story goes way back.
Sir Walter and Elizabeth learn that there were extenuating circumstances surrounding Mr. Elliot's marriage:
"Here was a great deal to soften the business. A very fine woman, with a large fortune, in love with him! Sir Walter seemed to admit it as complete apology;"
The fact that this completely explains Mr. Elliot's actions to Sir Walter's satisfaction and that it softens him makes me wonder whether this could also describe the late Lady Elliot?