I'm sure you know the answers to those questions but let me help others to understand.
Stevens was there to support and fund and provide arms to al Qaeda and other terrorist organizations to overthrow Assad in Syria.
He was unprotected because a large security detail would have drawn attention to what was really happening. Can't be covert and also visible.
The warehouses stored arms and supplies for al Qaeda to use.
Yes Stevens was supplying arms.
Obama saw things unraveling and knew that a major response would just shed light on the convert mission so he decided to eliminate the evidence (or allow the attackers to eliminate it for him) No witnesses and no one with details of what was happening.