Before the shooting, Dennis grabs Calvin's phone as they are walking down the street to apparently show him how to talk to women via social media. At no time does Dennis return the phone. Later, viewers learn via news broadcast and also see the scene where Calvin is shot after pulling his phone from his pocket.
When the main characters talk about light years they use it as a measurement of time while it is a measurment of distance.
Days after Calvin's funeral, CJ and Sebastian are at the gravesite where Calvin's headstone is visible. It takes weeks, more likely months, to get a headstone made, and is often unveiled at the one-year anniversary of the funeral. It would not be there within a few days.