I don’t think there was a kidnapping plot, not the way the terrorists set the main complex on fire. Stevens succumbed to smoke inhalation. And like the OP wrote, since caravans got held up all the time there, it would have been much safer to take Stevens captive while en route somewhere.
The complex was set on fire after several hours of fighting against the men who weren’t supposed to be there defending the complex.
Every agency that attempted to intervene was ordered to stand down and not interfere. Outfits specifically created to prevent such events were ordered to sit on their hands. Multiple requests and even demands to improve security at that complex here systematically refused.
That ambassador Stevens died of smoke inhalation was invented much the same as the video as an excuse for the attack.