Great questions, Riv! If I remember correctly, Tolkien originally wrote the tale for Aragorn to marry Eowyn. Arwen was a late-comer to the story, and by the time Tolkien conceived of the long-standing romance between Aragorn and his elven princess, most of the tale had already been written. Rather than try to shoe-horn the story into the already almost completed novel, he removed it to the Appendices, leaving only the few hints we have. I found the following in Letter #181:
Here I am only concerned with Death as part of the nature, physical and spiritual, of Man, and with Hope without guarantees. That is why I regard the tale of Arwen and Aragorn as the most important of the Appendices; it is part of the essential story, and is only placed so, because it could not be worked into the main narrative without destroying its structure: which is planned to be "hobbito-centric," that is, primarily a study of the ennoblement (or sanctification) of the humble.
Personally, I like your idea of inserting some portion of the love story into the tale during the time the Fellowship spends in Lothlorien. I'm sure Aragorn and Frodo had an interesting conversation on Cerin Amroth, after the hobbit witnessed Aragorn saying "Arwen vanimelda, namarie." After all, Frodo is the one who witnessed the look that passed between Arwen and Aragorn at the feast in Rivendell. Tolkien could have made us privy to that conversation, however, I suspect he felt it bogged down the flow of the larger story, and opted for the solution above.
Riv, I have a feeling the Professor WOULD have incorporate the story of Arwen and Aragorn into the main body of the story if it had occurred to him earlier on in the writing process. I have the feeling that by the time he'd worked all of that out, he was so far along in the tale that trying to reverse-engineer the entire thing proved to be too difficult. Hence, it takes pride of place in the Appendices.
I still remember my initial reaction after reading the following section of the Cerin Amroth passage for the first time:
"Here is the heart of Elvendom on earth," he said, "and here by heart dwells ever, unless ther be a light beyond the dark roads that we still must tread, you and I. Come with me!" And taking Frodo's had in his, he left the hill of Cerin Amroth and came there never again as a living man.
That last phrase left me with the horrible feeling that Aragorn was going to die at some point in the next few chapters!!! I was so stricken by the thought of losing my much beloved Aragorn that I went immediately to the end of the story to see if I could determine Aragorn's fate. You must understand -- I NEVER skip to the end of the book, but for Aragorn, I CHEATED!!!