Correct, the "I" mark for TDC is on the cam gear. What we need to determine is not where the cam is, but the piston position. I am trying to confirm that when the cam is in the TDCC position the piston is actually at the top of its travel. Hence, the straw down the spark plug hole and watching for it to change direction as the I ***** *****nes up. There is a small plug on the crank case cover that should have a mark for TDC as well.
As far as your question as to why the crank should not have to be held, think about the piston.rod at TDC. The wrist pin, crank pin and main bearing pivots are in perfect alignment at TDC and the air pressure is pushing straight down on the rod. Since there is no angle involved, the crank will stay at the TDC position without being held "IF" everything is correct. If it has to be held at TDC, it could very well be our problem. The procedure I am asking you to perform is confirming where the piston is. What you have done previously is confirm the orientation between the crank mark (flywheel) and the cam mark are correct. It does not guarantee the flywheel has not slipped or turned on the crankshaft.
So if the spark plug is coming out wet, with gas I assume, then it cannot be getting spark, as long as our flywheel hasn't slipped. Spark at the incorrect time will also prevent an engine from running, so we are back to confirming where the piston is when the spark occurs.
The way the picture is being painted, everything is perfect, but it doesn't run. Something is being overlooked, we just have to find it.