Trivia: Because they were both friends with Glenn Quinn, David Boreanaz and Christian Kane were uncomfortable filming the Lindsey-posing-as-Doyle arc.
Trivia: Joss Whedon wanted Sarah Michelle Gellar to guest star as Buffy in "You're Welcome" (the show's 100th episode) and "Power Play" (the penultimate episode), but she was unavailable. She later became available for the series finale, but Whedon declined, wanting the series to end focused on Angel. Conversely, David Boreanaz was available for the Buffy finale, but only for a limited amount of time.
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.