Others have touched on aspects of this in threads below, but I wanted to gather some of my thoughts into my focus on the backstory.

Of course the biggest chunk of it is at the start of Chapter 4, but I was actually surprised at how many other little clues are strewn here and there.

Some of the clues kept leading me back to Lady Russell and Lady Elliot. Here's what we learn about their backstory

--they were very close friends before Elizabeth Stevenson married Walter Elliot.

--Lady Elliot married Sir Walter as the result of a 'youthful infatuation'

--following her marriage, she was 'not the very happiest being in the world'

--Lady Elliot persuaded her dear friend to move near Kellynch and there relied on her kindness, advice and help in dealing with raising three daughters who had a "conceited, silly father"

--Lady Russell had "aristocratic ideas" and was "prejudiced on the side of ancestry" and "had a value for rank and consequence, which blinded her a little to the faults of those who possessed them." She had herself married "only a knight". Since she was free to move to the area of Kellynch when Lady Elliot asked her to, I think it is reasonable to assume she could have married before Elizabeth Stevenson did, and was widowed before she moved.

From all of that, I began to speculate. What if Lady Russell had had influence over her friend's 'youthful infatuation'? What if she (Lady R.) with all of her aristocratic ideas and having herself managed to snag 'only' a knight, persuaded her friend in favour of the handsome young baronet? Or at least said nothing to talk her out of it, being willing to overlook his faults because of his rank?

When the reality of the marriage became clear, and Lady Elliot realized that it was up to her and her alone to make sure there was 'method, moderation and economy' and to see that they lived within their means, and to conceal Sir Walter's failings she prevailed on her dear friend to come and help her.