Let's recap; It will turn over by hand with the glow plug in, just not with the starter. Correct? If so, I'm assuming that the piston is not bumping the bottom of the glow plug.
First, from what you've said, I don't think it'll have anything to do with the carb or the needle valve or trash in the line or anything of that nature. Besides, the only possible relationship that could exist there is if you're flooding it and you've already said that you cleared it by removing the glow plug and spinning the fuel out of it right? It's normal for the engine to be much harder to turn over with the glow plug in place. With it removed, there's no compression so naturally, it's easier to turn.

Questions- Do you have any experience with hand starting? If you can get it started by hand then you can verify that the engine itself doesn't have a problem.
Have you checked the voltage on your starter battery? Is the starter cone just spinning on the spinner or is it coming to a stop, unable to turn the motor past compression?

Just trying to ask enough questions to give us all some clues as to what the trouble is.