Some executives will ask for the same signatories for both the Author's Panel and Q&A, while others may allow, recommend, or require that the delegates change. Ask your chair about their policy and discuss in advance.
The time allotted for Q&A is the amount of time unused by the author's panel. Time will be paused upon conclusion of the presentation, and will resume only while signatories answer questions; the timer does not run while delegates ask questions.
Think of the Q&A as similar to a press conference. You need to give a good impression to garner support for your resolution, but ultimately the length of your answers, the questions you accept, and your approach and tone are up to you.