I don't know if they exactly planned it, but they apparently knew about it ahead of time and went with it, assuring that security would be woefully insufficient. I think the motive can be found in Biden's remark in the debate that (I don't recall the exact words) it "used to be" that everyone would unite (behind the administration) after an incident like this. I.e., all rally round Obama in this crisis less than a month before the election!
You’re probably right about their motivation. However, the “rally around the president” phenomenon only works when the people don’t blame the president for creating the situation in the first place. Even if the administration had been successful in covering up the fact that this was an act of terrorism, enough people disagreed with his “Arab Spring” policies that I don’t think the administration would have gotten away with it. The first thing I thought when I heard our ambassador had been killed was that it was Obama’s fault for helping to install the Muslim Brotherhood into power in Egypt and Libya.