The call to adventure for Scarlet was when she was kidnapped by monk-like people. She had no choice but to comply with the wishes of the monks, since she would have been killed any other way.
Crossing the threshold for Scarlet was when she learned that she was a gatekeeper. She didn't have much of a choice, still. You could say she was destined (this idea is also supported by how she's been having dreams of the other gatekeepers).
Scarlet had the other gatekeepers as helpers. Although this happened earlier in the story, her first test was when a mysterious man saved her from getting him by a car before leaving. She is taken away to Hong Kong and the rest of gatekeepers, including the protector, have to rescue her from the Old Ones (the monks).
The initiation for Scarlet doesn't happen in this book, but in the next. A close part of her initiation in this book is when she is escaping "The Needle", where she was imprisoned under in Hong Kong.
The main flight and return also occur in the next book. Scarlet "returns" when she manages to get out of the The Needle, thus getting away from the Old Ones that were there to stop her from leaving. She escapes from Hong Kong and the Old Ones are left to gather their forces while the Gatekeepers plan their next move.