Hi Adam,
The vac on snap throttle sounds about right but not sure about the bouncing when the engine is hunting. I would think that's just a symptom?
The 18" when idling and aircon switched on seems a bit low but I have seen engines that are running perfectly show similar readings. Their is plenty of info on the net on good and bad vac readings so might be worth a look. If your boss want's results he needs to make some investment. A smoke machine is not what could be called a major outlay and it would have confirmed or eliminated an air leak in minutes. See if you can borrow one so he can see what it will do (your Snap On dealer may know someone that could let you have one as a demo). Shane's suggestion also sounds viable.
Bryan.