I don’t think Lady Elliot did see anything special in Sir Walter after the glamour wore off, unless it was especially bad. (;D) In Ch. 1:

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.

I think it was probably a jolt to realize she had married a vain, selfish, superficial ninny—put it down to the blindness of infatuation. Afterwards she “humoured, or softened, or concealed his failings, and promoted his real respectability for seventeen years” which indicates to me she realized his failings very well. It seems she realized her error but chose to do her duty by her husband and try to enjoy other facets of her life, “though not the very happiest being in the world herself, had found enough in her duties, her friends, and her children, to attach her to life” but despite all she did for him Sir Walter did not value her. In Ch. 17 when hinting of Anne marrying Mr. Elliot and taking her mother’s place at Kellynch she says, “…only superior to her in being more highly valued!” I think Anne is a lot like her mother. She fell in love rather quickly, somewhat like an infatuation, but Frederick had and has substance unlike Sir Walter. The deep stuff Anne sees in Frederick actually exists. I don’t know if Anne was luckier than her mother or just more observant. It is difficult to imagine any infatuation coloring Sir Walter’s personality to the good side. Did they never speak before marriage? (;D)