Are you sure that fuel is actually getting through the NV assembly? Cleaning the needles is a good thing to do, but did you clean out the jet? Try passing a "cats whisker" wire through the jet, or try blowing through it.

You probably do need the backplate sealed. Use a thin film of silicone sealant on the mating surface of the backplate.

Try removing the muffler & squirting a bit of fuel into the cyl through the exhaust port, then attempt a start. If it fires, you are simply not getting any fuel into the crankcase through the needle valve assembly.