I think that having a fake Buffy assigned to Rome for some mysterious reason was just something that was tossed into Season 8 so that there wouldn't have to be any sort of explanation as to why the real Buffy was no longer there. Joss just didn't want to write the story that way. The same goes for the deus ex machina that got Angel, Spike, and Illyria out of that alley after "Not Fade Away," and even why Willow revealed that Buffy actually knew that Spike had come back, and where he was, after "Chosen," Joss just plain didn't want to be bothered with writing any sort of real story clearing those things up. And, frankly, I was, for some reason, really intrigued with the mysterious character known only as "The Immortal." I really wanted to see just what, exactly, he was, and what the power that he seemed to have over everyone, especially women. Then, once having found that out, I really wanted to see Spike,Angel, or both of them, kick the immortal crap out of him.