One Luh's rapping was actually written and recorded by actor Justin Lee over instrumental tracks by Ryan David Tsang, who played One Luh's music producer, Rory, in the film.
The hands making the margarita in the beginning of the restaurant scene are actually the hands of actress Amanda Kathleen Ward (Lena). This symbolizes that Lena feels Rory's accident is on her hands.
During the accident scene, all the EMTs are actually real-life professionals, not actors. They were directed to react with the same procedures and terminology they would use during a real first-responder situation.