The answers are:

1. After you filing for changing the status,that is the I-485, you lose your F1 status. Your status now is AOS.
2. If the case is pending, you can cancel it. If the case is denied, you can not cancel your case anymore.
3. If you cancel your application before INS make any decision, that is when it is pending. you can resume your F1 status. And you can return to China and visit your family.
4. If you case is denied by INS, you lose your status, and if you return to China, you can not get back to US.
Good luck.