I am a believer in filling ANY carburetor that has been off the engine prior to starting.

I have never been concerned about dirt into the vent; but if you are, a piece of masking tape covering the hole until you are ready to start the engine. DON'T FORGET TO REMOVE THE TAPE wink

Will leave your other questions to others.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop