You're right. I thought I had finished the content when it first asked me for IAP, but you get to continue fighting bosses even if you can't unlock them. You are stuck playing the same two songs for non-boss levels since the levels leading to boss fights are based on your character. For $.99 it's not bad. The way they require IAP to unlock demons you have already defeated is horrible business, so I'm not giving them any more money.
I'm gonna have to call foul on finishing all the content in under 30 minutes. Maybe you finished easy mode in 30 minutes, but normal and hard mode have additional stages and hard mode is a lot more difficult. It's a pretty decent value at $0.99, though the IAP isn't really worth it.
I just stick with the original character. Her stats are lower, but it's better than man voice :/
Totally agree about the voice thing though.