Why did Nick not play on Act II?
TDH band members are a revolving door. There's like a new line-up every week. I think I've seen them live with 4 different lineups (and a 5th for the show I tried to go to but couldn't get tix for :/).
I think when Casey did Act I, he didn't really have a "band"... he just used friends and family members to help him out, which is why Nick played on it. By Act II he had started putting together an actual "band", which included the Dent brothers. But then they turned out to suck, so when he needed a new drummer, he was like "Hey, my brother is a drummer, maybe he wants to be in the band for real". So its more that he wasn't actually a band member at first, rather than doing a "leave then come back" sorta deal like Josh.