Actually, those are moments about the same position, to a first approximation.
If you assume:
That the thrust vector from the live engine is along the centreline; and
The aircraft cg lies on the centreline
Then the moment due to the engine relative to any point on the centreline is the same - it's the net thrust of the engine times the lateral offset of the thrustline from centreline.
Therefore whether the c.g. moves forward or aft does not affect the thrust moment - only lateral motion of the cg would make a difference.
Similarly, since the sideforce generated by the rudder is considered to act purely sideways, the yawing moment due to the rudder does not vary with lateral motion of the cg - all that matters is the distance from the point of action of the force to the cg, meausred at right angles to the force.
Now, I'll grant that in practice the thrust line is often slightly canted in or out, and that the cg is not perfectly on the centreline, and the real direction of the rudder force may even be slightly canted (though by definition that is accounted for in another force component). But these effects are largely secondary to the main effects.
With regard to moments of inertia: if looking at a simple force/moment balance, then inertia terms don't actually matter - rudder moment = engine moment is a calculation that can be done independent of inertia terms. Where inertias come into consideration are dynamic events (and Vmcg can certainly be dynamic in real life). Although Vmcg is often tested at light weights, the actual reg states "most unfavourable weight"*, and AC25-7A suggests "VMCG testing should be conducted at the heaviest weight where VMCG may impact the AFM V1 schedule." My impression has always been that weight/inertia is at best a second order effect on VMCG - while the heavier aircraft is slower to diverge, it's also slower to return to centreline and the two effects are closely cancelling.
* Incidentally, the current FAR 25.149(c) states that VMCL uses " (4) The most favorable weight, or, at the option of the applicant, as a function of weight;" which is certainly an interesting typo!