I am not sure about your setup but if you are getting significant air out of a different cylinder (youtr #5 results) it seems likely that the air is going past the piston into the crankcase and then coming out the most convenient cylinder. Your results for cyldiner #6 would indicate the exhaust valve wasn't seated or is leaking a lot but that doesn't really fit your exhaust smoke and oil fouled spark plug. I suspect your setup is off but your general description of positioning the engine such that both valves are closed should be correct.
I'm sure someone else will give you a better answer.
