Yep, very possible. With the stock inlet functional, a leak between the crankcase and intake would act as a slightly stuck PCV valve, the air is already metered and accounted for, so the IAC takes care of keeping the engine at the right RPM. With the inlet blocked and the breather on, it would be a vacuum leak.
With both the inlet and outlet blocked, you should have positive pressure on the front valve cover, not negative. For the IC to be at fault here, it would have to be a serious design defect, such as the core extending all the way into one of the PCV passages.