I think your right—young Miss Stevenson was blinded to Sir Walter’s character. Youth and infatuation are two commodities particularly dangerous to domestic felicity in The Novels! Switching the sexes, Lady Elliot like Mr. Palmer (S&S) found that through “some unaccountable bias in favour of beauty” (S&S, 20) she was the wife of a very silly man:

His good looks and his rank had one fair claim on his attachment; since to them he must have owed a wife of very superior character to any thing deserved by his own. Lady Elliot had been an excellent woman, sensible and amiable; whose judgment and conduct, if they might be pardoned the youthful infatuation which made her Lady Elliot, had never required indulgence afterwards. (1)

The mechanism was very similar to Mr. Bennet who was “captivated by youth and beauty, and that appearance of good-humour which youth and beauty generally give” (P&P, 42). In my opinion Lady Elliot handled her situation much better than Mr. Palmer or Mr. Bennet whose treatment of their silly wives deserves some criticism. Lady Elliot had “humoured, or softened, or concealed his failings, and promoted his real respectability for seventeen years” (1). I imagine it was a trying, never-ending thankless task. It is no wonder she was “not the very happiest being in the world” (1).