You seem to be suggesting that Marianne and Willoughby’s relationship was a flirtation and had Marianne bucked up with some common sense she would not have been so hurt. I cannot say there was no error in Marianne’s behavior but I do not think there is any assurance she could have saved herself heartbreak with common sense or a strong Lizzy-like character. Marianne, should have been prudent, hid or moderated her early warmth, protected her heart and not let her feelings run wild especially in the beginning of their relationship but unfortunately it was not in her nature. However I think Marianne can be faulted only so far.

Marianne was also guided by Willoughby’s behavior, as she was supposed to be, and taken in by what it proposed. Willoughby sought Marianne out, pursued her, and continually raised the seriousness, intensity, and intimacy of their relationship with his behavior and sometimes his words. Willoughby sent all the right signals, his overtures, accept for a final declaration, the intent he wised to convey could not have been clearer as he admits: “the absurdity, and the worse than absurdity, of scrupling to engage my faith where my honour was already bound …openly assure her of an affection which I had already taken such pains to display” (Ch. 44). The problem is Willoughby’s behavior was not to be trusted because he was misusing courtship rituals to create a stimulating situation for his amusement. There were clues to Willoughby’s deceit early on but I do not know that Marianne is particularly to blame for not seeing them.

I do not know if I would classify Lizzy & Wickham’s relationship as a flirtation (that is a question for the P&P board) but I do not classify Marianne & Willoughby’s relationship as a flirtation at all—especially as you describe it “a bit of fun, between two well matched individuals, an exchange of compliments etc which may lead to a serious love relationship or which may fizzle out after a while” (Post flirting). First neither Lizzy nor Marianne, despite their intelligence and understanding are on equal terms with Wickham and Willoughby because they are being lied to. Second flirting is not a bad thing when done appropriately, without ill intent and with propriety. I think modern and Georgian views on flirtation are not the same.

There is also a great deal of difference in the makeup of the relationships between Lizzy & Wickham and between Marianne & Willoughby. Lizzy believed Wickham preferred her but she had no reason to believe he had serious intent towards her or that they were engaged so it was not just Lizzy’s common sense and a strong character that protected her heart. It was also the fact Wickham did not act the same as Willoughby. He did not take up the intimacies of an engaged man as Willoughby did. Wickham did not call Lizzy by her first name, he did not beg a lock of hair, kiss it, and fold it up in a piece of white paper, and put it into his pocket-book and he did not give her an expensive gift, romantically named horse, he never insinuated her future married state—Wickham never spoke to Lizzy as intimately as Willoughby did to Marianne:

His concern however was very apparent; and after expressing it with earnestness, he added in the same low voice -- "But, Marianne, the horse is still yours, though you cannot use it now. I shall keep it only till you can claim it. When you leave Barton to form your own establishment in a more lasting home, Queen Mab shall receive you." (Ch. 12)

"There certainly are circumstances," said Willoughby, "which might greatly endear it to me; but this place will always have one claim on my affection, which no other can possibly share."

Mrs. Dashwood looked with pleasure at Marianne, whose fine eyes were fixed so expressively on Willoughby, as plainly denoted how well she understood him.

"How little did I then think that the very first news I should hear from Mrs. Smith, when I next came into the country, would be that Barton cottage was taken! and I felt an immediate satisfaction and interest in the event, which nothing but a kind of prescience of what happiness I should experience from it, can account for. Must it not have been so Marianne?" speaking to her in a lowered voice."