Hangouts On Air will move from Google+ to YouTube Live effective of September 12. Events cannot be scheduled on Google+ after September 12 and you will need to move existing events scheduled to happen after September 12 to YouTube Live.
YouTube Live has built-in events scheduling that lets you schedule your event for a future date and time. This feature is similar to Google+ and Calendar integration for Hangouts On Air.
However, Google in its blog post says, that recorded Hangouts On Air will still be available on YouTube, and that Google+ will have your event content available in read-only format in the Activity Log.
In regards to Q&A, Google’s suggestion to broadcasters is to use Slides, and questions to be gathered ahead of time via social media platforms.