RE: CG Setup and fuel level?
An Aircraft is always balanced for the worst condition.
If the tank is ahead of the Centre of Pressure then the tank should be empty.
If the Tank is behind the Centre of PRessure, then the tank should be full.
In both conditions this will result in a Centre of gravity (CofG, CoG, CG)that is ahead of the centre of pressure, resulting in a stable airplane.
Now wrt to the tank being mounting with its butt end against the wing tube, no problem as long as you have a tank that is pressurised by exhaust back pressure. AS Long as there is a positive pressure in the tank and a negative pressure (lower than 1Atm) in the carb then fuel will be drawn toward the carb. For 3D type airplanes it would be wise to consider a pump or sorts.