Dirtybird
Last Imac event I went to had a fellow there flying a small plane with a 55 AX glow engine in it. He was competing just like the big boys.
As far as a pump adding pressure to existing pressure depends on which end the pump is fitted onand of course the type of pump used, our pumps on the carb on our engine's are used to suck the fuel not put pressure in the tank like a glow engine does. Nor is the pump in the tank so it has to push the fuel, in all cases in a gas engine with a walbro or similar type carb the gas is pulled.
The small amount of pressure that's created when the vent line is inverted is nothing as compared to what that small pump can pull. Some one said 18 Inches A good pump will pull 3 feet straight up.
I myself use a 3 line set up with the filler line plugged after use and have never had an issue of any kind.