Hi Shabazza,
you are right in that the fuel load does affect the CG. As does the payload.
We - for the IXEG - try to simulate what the pilot does...which is regularly not determining how the payload is distributed, pax seated etc. The pilot simply gets a loadsheet which shows him the CG for takeoff (with the fuel being at take-off fuel level) and the CG for the zero-fuel-weight (which would be where the CG is after all fuel is burned). He (double-)checks wether this is "in limit", then enters the takeoff CG into the FMS and sets the trim accordingly. Thats it.
So when you fly the IXEG 737, imagine that the "ground handling" guy gives you those figures (as a printed loadsheet). You simply punch in the numbers.
I understand what you are saying - if you have a certain CG and NOW change the fuel, the CG should change...but think of it this way: You have an aircraft with at CG of 23% MAC at 100pax and 6.0tons fuel. Now you decide to take on another 2 tons - so now its 8.0. CG should change...but it doesn´t because suddently 20 pax from the back move to the front - voila, CG stays the same - even though fuel changed!
What I am saying is that everything influences CG. Ultimately the pilot gets the CG for takeoff - and in our simulation you get to determine the CG yourself. It is not "calculated" by simulating fuel and weight distribution. Just like in X-Plane, you simply set it where you think it is. If you want to, you can use the real load+trim sheet for a 737-300 and figure the CG out for yourself, of course.
Cheers, Jan