What you describe sounds like the existing Communities feature, but until there is code block formatting in posts it'll still be a pain to read code in G+
I'm missing why it's not replacing the G+ Messenger app (or maybe it is, idk). For some one who's been able to install the new Hangout app - do you also still have the Messenger app as well?
How do you go about doing that? Just decide on an amount upfront manually watch the balance? I didn't see any function to cap the total lifetime spend on a campaign or stop it once it hit a certain level.