Sorry about the dead end.
The pipe going to the bottom of the crankcase is a scavenge pipe that removes excess fuel that gathers there. It could have a non return valve inline so will not be able to blow down but probably could suck?
The only thing that I can think of (going off what you say) is that the idle jet is blocked, and by holding the throttle butterfly open you are drawing fuel from the main jet.
As you can appreciate it is difficult to diagnose without being able to see or hear what is going on.
I will keep thinking about it and if I come up with any more bright ideas
I will let you know.