Stevens and the CIA were running weapons from Qadaffi’s stores to ISIS in Syria via Turkey. Stevens was in Benghazi to meet a Turkish attache regarding a shipment the day of the attack.
My guess for the reason for the attack is simple: Libya jihadi militias wanted those weapons for themselves, and weren't too keen on having them taken away.
The whole operation was on a need to know basis: the operation was run from Benghazi because it's more isolated. In Tripoli, these activities would have drawn too much attention.
The attack when it occurred was monitored live in the WH, but no rescue operation was going to be authorized - the election was a couple of months away and any rescue would draw attention to what was going on there. And a jihadi attack on 9/11 would give the lie to Barry's foreign policy “successes.”
The guys in the field were left to fend for themselves, while the WH cooked up the stupid video story.
Barry figured that a compliant press more interested in getting their man reelected would ignore the story, and he was right.
These military guys in Tripoli probably suspected more than they're telling, but since they were kept in the dark about the operation they can testify under oath that they didn't know much and still technically be telling the truth.
Excellent summary!