Michelle Branch, the singer who performs at P3 during the first part of this episode, also makes an appearance a little over a year prior (Nov 2001) at The Bronze in Buffy the Vampire Slayer (1997) Season 6, Episode 8 "Tabula Rasa" singing the same song, "Goodbye to you". Many of the same song acts appear in both shows throughout each series.
Way back in 2002, when there were rumors that a member of the main cast was leaving Charmed, there was some sort of Red Herring, because some people thought it was Brian Krause the one departing from the Show, but in reality, it was Julian McMahon, to star his own show -Nick Tup-.
There are many theories about who could the Darla of the alternate reality. Some believe is the Succubus from season two. Others believe she could be a demon stripper like Kaia. Others think she could be the same type of Demon than Julie {Deborah Kellner} from season 4. The less likely seems to be the Succubus, because that would create a plot-hole.