Haven't read through all this so don't know if it has been mentioned before.
To clean the float needle seat I put a Q-tip in a drill, dab some brasso on the end, stick it into the seat hole and polish the seat on a low-ish speed.
Flush it out completely (I prefer using iso-alchohol as a flush) and clean or replace the float needle.
Polishing the seat this way provides a uniform and smooth surface for the float needle to seat against.
Once you've adjusted the float level and polished the needle seat there's no reason it should leak.
The float bowl gasket is meant to stop leaks while the moped/carb is temporarily tilted (as in going around a corner or similar). It is NOT meant to stop carb from leaking if too much gas is getting into the carb. Nothing will stop that. You have to prevent too much gas from getting into the carb in the first place.