I think you are right about Janet Fraser’s marriage being a warning because she married for the wrong reasons. The parallels you made with Janet’s and Fanny’s advisors is interesting. It seems all who advise Janet share her materialistic values per Mary: “it was a most desirable match… We were all delighted. …he was rich, and she had nothing” (36). The fact Janet asks for advice suggests to me she had some reservations against Mr. Fraser but being mercenary and ambitious and supported by her like-minded friends she accepted his offer. Fanny’s independence of her advisors brings to mind what she tells Henry in Portsmouth: We have all a better guide in ourselves, if we would attend to it, than any other person can be (42). It seems, unlike Fanny, Janet did not listen to her inner guide and perhaps that adds to her bitterness:

She is a cold–hearted, vain woman, who has married entirely from convenience, and though evidently unhappy in her marriage, places her disappointment not to faults of judgment, or temper, or disproportion of age, but to her being, after all, less affluent than many of her acquaintance… is the determined supporter of everything mercenary and ambitious… (44)

I feel each Ross sister’s marriage is a warning on the importance of character and compatibility towards happiness in marriage. This I think applies to all the mismatched couples at MP but I think it is particularly aimed to showcase Mary’s choice because at it is she who waivers between accepting Edmund for his good qualities as a man and rejecting him because of his profession and lack of fortune. Mr. Fraser “turns out ill–tempered and exigeant” (36) which is probably due to differences in age and temper as Edmund suggests but whatever the cause it seems sure neither Janet or her friends foresaw the problems. Mary’s failure to understand poor values and inconsideration of Mr. Fraser’s character sabotaged her friend’s happiness leaves Mary in jeopardy of making the same mistake:

Poor Janet has been sadly taken in, and yet there was nothing improper on her side: she did not run into the match inconsiderately; 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, and especially applied to my late dear aunt… and she was decidedly in favour of Mr. Fraser. This seems as if nothing were a security for matrimonial comfort. (36)

I can see Mary’s poor values at work in her initial judgment of the Mr. Bertrams. She very quickly and superficially decides she would accept Tom despite the fact it seems all she knows of his character is rather shallow. He was the ‘sort of young man to be generally liked’ (5) with the kind of agreeableness ‘was of the kind to be oftener found agreeable than some endowments of a higher stamp’ (5) such as sincerity, steadiness and integrity perhaps? Her disinterest in why his father extracted a promise to limit gaming (5) could result in finding herself married to a selfish spendthrift. Tom’s worth is his ‘easy manners, excellent spirits, a large acquaintance, and a great deal to say’ (5) and ‘the reversion of Mansfield Park, and a baronetcy’ (5). When Mary finds Edmund agreeable she is a bit puzzled:

…without his being a man of the world or an elder brother, without any of the arts of flattery or the gaieties of small talk, he began to be agreeable to her. She felt it to be so, though she had not foreseen, and could hardly understand it; for he was not pleasant by any common rule: he talked no nonsense; he paid no compliments; his opinions were unbending, his attentions tranquil and simple. There was a charm, perhaps, in his sincerity, his steadiness, his integrity, which Miss Crawford might be equal to feel, though not equal to discuss with herself. (7)

After Tom returns to MP in September to be ‘gay, agreeable, and gallant again’ (12) Mary discovers ‘by the power of actual comparison, of her preferring his younger brother’ (12). It seems she began to value character over ambition but never completely learns the lesson—her regard for Edmund creates a kind of war within which never allows her to accept his profession and merely adequate income. It seems like Janet, Mary does not know how to “manage him [Edmund] well; she does not seem to know how to make the best of it” (36) or rather accept his fortune or profession. All her efforts to change his profession come to naught and she certainly, like her friend, sometimes displays a “spirit of irritation” (36). After Mary returns to town and the influence of Mrs. Frasier rank and riches again dominate Mary’s view of a husband.

I have not so much to say for my friend Flora, who jilted a very nice young man in the Blues for the sake of that horrid Lord Stornaway, who has about as much sense, Fanny, as Mr. Rushworth, but much worse–looking, and with a blackguard character. I had my doubts at the time about her being right, for he has not even the air of a gentleman, and now I am sure she was wrong. (36)

I think Lady Stornaway’s marriage was also an example for Mary—one she misreads at first. Either she was very lucky or Flora Ross considered character and compatibility in her choice of husband more than her sister. I infer this from Flora’s happiness and Mary’s changing view of Lord Stornaway. Mary initially felt choosing him over the “very nice young man in the Blues” (36) was a mistake. It seems she judged Lord Stornaway superficially. She believed him rather stupid and ungentlemanlike (see above) and may have based his blackguard character on what she saw as a disagreeable countenance. I imagine the nice young man in the Blues possesses a very fashionable manner and she certainly finds the Blues (Navy?) a socially acceptable profession. Mary made this point to Fanny and Edmund:

“The profession, either navy or army, is its own justification. It has everything in its favour: heroism, danger, bustle, fashion. Soldiers and sailors are always acceptable in society. Nobody can wonder that men are soldiers and sailors.” (11)