Cees the first drawing of the RC tank should have the venting air line inside of the tank attached to a piece of bent metal tubing. This positions the end of the vent tubing right next to the top of the tank and prevents the siphoning of the fuel when the tank is full. Ed's tank with the red band seems to be a two tubing tank - one to the motor and one for venting. As you can see the venting line does have the bend to the top of the inside of the tank. It looks as if the metal tubing outside of the tank broke off. The flow to the motor, as your drawing shows, is dependent on fuel position relative to the needle valve.

If the tank is level it doesn't matter where the vent is - the vent just bubbles until you have a constant pressure inside the tank. If you are maneuvering the internal pressure varies a lot but apparently the motors can handle it fine - most modelers just flop the vent over the side or leave it stuck up in the air.. You can put the vent forward but the pressure varies a bit too much.

Making the venturi smaller certainly eliminates some of the sensitivity of this operation. Certainly Ed seemed to go for the least complicated solution for problems. He probably fueled through the line to the motor if it is a two line tank.

I had a bad inner ear problem so never got into control line but I remember reading about a "chicken hopper" fuel tank design one time.

Ben