No, you don't have to pay the excess fee as long as you can provide name and contact details of the driver who was at fault. The case then will become the business between AAMI and the other guy.
Insurance companies are very sensitive about this. Both you and your friend may need to explain why you are the second driver of your own car. For this particular case you are not at fault, and the loss could be recovered from the other party, therefore the insurance company wouldn't normally spend too much effort varifying the details, HOWEVER, since your friend is the first driver on the policy, which means this policy is actually under his name, he will be fully liable to this policy. That means, if you have an at fault claim in the future, his insurance rating will drop with it.
It is a serious offence to declear fault information to insurance companies. They can reject your claim or even cancel your policy if they find out about it. If you have an accident on the road, without insurance company protecting you, you are fully liable for the property damage yourself, not your friend. If the accident involves driving offence and someone is killed, you may also face criminal charges. Nothing to do with your friend.
4. 如果处理完这件追尾事故后（责任不在我），可以要求AAMI变更第一驾驶人吗？
Yes you can, just speak with them ove the phone and inform them that you have now become the first driver in the policy. You may have to cancel the existing policy and take out a new one.