RE: CG Calculation for Mystery Plane
Your guess is correct.
For proper location of the CG, the MAC location of the semi-wing and the tail must be calculated.
When calculating the MAC for the wing, enter S=0 into the calculator, since the imaginary equivalent straight semi-wing should be located closer to the leading edge.
When calculating the MAC for the tail, enter S=value of sweep back at the tip.
For a leading edge that is actually swept forward (not straight like yours), the value of S should be negative.