OK, Jim. If the question is,
"Could either D or E fall in love with someone other suitable before they were definitely in love with each other? (and in Darcy's part, I'd put it at Hunsford, on Elizabeth's, sometime afterneading the letter butnat the latest, their Pemberley meeting) - the answer is, yes. I haven't read Peter's story only the beginning, but yes, it's a plausible story.
If the question is what the topic title says, "would it be an advantageous marriage in the material sense, would everyone leap at the chance to marry each of them?", then Darcy is a catch inspite of his faults (most women would overlook them, and in the end, Darcy would not treat a wife of his badly), and Elizabeth is not (her beauty is nothing out of the ordinary, her intelligence and wit aren't primary values in a wife according to the ideas of the era, and her unconventional behavior is a liability - and she has nothing to compensate for it in the material area).
But if the question is, "are they really so great together, with all the faults they bring into the bargain?" - then my answer is an emphatic Yes! The reason (one of them, anyway) why I love P&P so much is exactly the fact that they complement each other, bring out the best in each other, are equals in spite of the social/fortune difference.
