Question: I know the real world reason Bruno Kirby wasn't in this one (horse allergy that had to be kept at bay with daily medicine shots in dosage size and delivery needle fit for a horse), but is there any in-movie explanation for his absence here?
Question: What song were Mitch, Phil and Glen humming before Duke to told them to stop?
Answer: "The Godfather Waltz", the main title theme to the Godfather film. It's a reference from early where Glen is a fan of the film.
Answer: No in-movie explanation has been given but, at the end of the first movie, Ed said that he was going to get Kim pregnant so, it can be assumed that Ed decided to become a dad and spend time with his family.