I read that section in Chapter 36 through a couple of times, because it occured to me that it could be taken a different way than that which I had first read it (likening Janet Fraser to Maria and Mr. Rushworth). I began to be clued in where it reads:

"[A]nd yet there was no want of foresight. She took three days to consider of his proposals, and during those three days asked the advice of everybody connected with her whose opinion was worth having..."

Janet believed she was right in accepting a "most desirable match" due to his wealth and influence and the opinions of anyone who had one supporting it. Fanny also receives no shortage of opinions, all encouraging her to accept Henry's offer of marriage. Even Edmund, the person whose opinion she most respects (like Janet's late aunt), tries to convince her to marry Henry. I think JA is allowing us a hint of what might've happened with Fanny and Henry had she listened to everyone besides herself. In short, they would be "about as unhappy as most other umarried people," in marrying for the wrong reasons.

Robbin, please correct me if I'm misunderstanding you, but I read the situation regarding her friend Flora exactly opposite as you did. I understood it as, "I [I]had[\i] my doubts at the time about her being right [in accepting Lord Stornaway in place of the young man in the Blues], for h3e has not even the air of a gentleman, and now I am sure she was wrong" [and indeed is sure that her friend should not have married Stornaway].