Nathaniel was given a lot of screen time since his first episode in season 2, and Josh was pushed into the background halfway into season 3. Second, Josh was clearly never intended as the endgame love interest or a lead character, with the writers always being addement that Paula was the second lead.
This episode make it clear how the writers have struggled to find a place for White Josh, given how Maya, Jim and Tim have all done more singing on the show. WhiJo not directly being a friend of Bex or a a co-worker means he had no place in this episode or last week's.
Getting Over Jeff was a Paula-centric episode, with Bex and Josh having smaller roles.
That was part of the gag. Marcy had a squeeky voice in the dream (and kept saying "I'm Marcy") based solely on Maya's impression of her, Greg's douche personality was purely because Nate was jealous, and Bex constantly telling Greg he is funny was because Nate heard them laugh.