I agree with much of what you are saying, except that Mr. Elliot's late wife was described as being of "inferior birth"--not 'low birth'. We discussed this earlier in the read, and I believe that this 'inferior' might only be in reference to the fact that she was 'inferior' to Elizabeth Elliot (the daughter).

Lady Elliot was from an inferior rank to Sir Walter because the baronetage lists her as the daughter of "James Stevenson, Esq. of South Park". Her father was not a baronet or of any other title, so in that sense she was of "inferior birth" to her husband. I suppose there could be an earl or a baronet farther back in the Stevenson family tree, but it seems that Elizabeth Stevenson was a gentleman's daughter and nothing higher.

If she were from a higher or equal rank to Sir Walter, then her family would have its own entry in the baronetage, and her marriage to Sir Walter would be recorded there too. So there would be two entries that Sir Walter could fondly gaze upon, but there is no mention of this.