Hey mate, seems like you have most of the bases covered. Usually the best way to test for leaks is to pressurise the intake.
If you are handy with a welder, weld a compressor fitting onto a piece of pipe with a tap, and weld the end shut. Them simply remove your AFM and clamp it there and slowly crank the pressure up to 30psi.
It is by far the only accurate way to find leaks.