That business of removing and cleaning the glowplug is one of thos things with inverted engines. Oil runs into the bottom of the cylinder if the model is stored upright, as you noted. Nothing to do with the pump, IMHO.

Perhaps you need to put you hand over the muffler when you flip it through to prime it. You CAN force fuel the VP-20 if you try hard enough. In a nutshell, I suspect that you are just not getting it primed right to start it after you blow out the plug. Also I have a feeling that you might need a bit more pump volume so that your engine's needles can be in control. Keep in mind that you want the pump volume adjusted so that the needles can make the engine run over-rich. You may have it a bit lean overall.

CR