To resolve the battery drainage issue please follow the troubleshooting steps provided in the given link: http://www.samsung.com/in/support/skp/faq/1101689
Further, we have noted down your feedback regarding the battery drain because of adaptive brightness and any update in this regard will be conveyed to you through various means of communication.
Team Samsung Members
Yes, front camera is the ambient light sensor, also I think it's not safe to let camera open in background for light sensing. It conflicts with privacy concerns. Please have a look Samsung.