Billy = Adam DeVine (I low key think his persona is influenced by Billy, Adam Demamp seems drawn from him too, and they all share a derpy physicality I would say)
Mandy = Aubrey Plaza? Doesnt “look” like Mandy at first but April from Parks and Rec seems very Mandy
Aubrey Plaza came to mind for Mandy before I read the rest of your post. Though given Mandy’s age, I’d also like to pitch Jenna Ortega.
Who’s Grim? Adam Sandler? /s