There are definitely shades of Charlotte Lucas in the way Lady Elliot "managed" her husband. Though she didn't enter into marriage with the clarity Charlotte exhibited, she made the best of it.

I think Elizabeth fell in love with Mr. Elliot, and the idea of marrying the heir of the family was too irresistible. After that fell through, she still wanted to be sought by "baronet blood" rather than someone lower-ranking. As to why she did not set her sights on marrying into nobility, perhaps they did not move in the same circles?