Well, I don't think you could put the insulator or gasket on upside down without it being very obvious. I don't understand the part about there being a flat part next to the bolt hole.
It seems to me that the next step is to lay each of the other parts (gasket and insulator) on the carburetor flange, and see if the three holes in the flange are unobstructed. You might also try clamping those two bits to the flange and verifying that the engine runs properly. Then we'll know it is the elbow that is at fault. I'm conscious that one of the moulded recesses in the elbow is very much smaller than the corresponding recesses in the gasket and insulator, and I think it won't reach across the matching hole in the carburetor. Does it actually block the hole? That may depend on how it fits over the gasket and insulator.
If it runs well with gasket and insulator clamped in place, the next step would be looking at the gasket and insulator placed over the flange on the elbow, to see how they match up.
