It's not immediately obvious to me what your question is....
If you are asking why you can't apply the inertia relief of wing fuel to the ZFW limit:
Assuming that the max ZFW limit is being imposed by structural considerations, perhaps wing bending loads at the root, then adding wing fuel will not reduce the requirement to carry the fuselage weight throught the root. The load at the root will not go up (because the additional mass has been added to the wings as fuel, and so inertia relief applies to that added mass) but neither will it go down - the weight of the fuselage at 1'g' is still the same, and must still be transmitted through the same load paths.
Inertia relief from wing fuel enables you to carry a higher total aircraft mass than would be the case were the fuel carried in the fuselage; it does not transfer the loads for the fuselage into the wing 'for free'.
Regarding the statement of 'never having no fuel in the wing'. If you were to use the reserve wing fuel and assume it was always in the wing, to meet some loading restriction, then you should not be counting it as reseve fuel - essentially that has to become unusable fuel. Otherwise you may get into a situation where you need to use the reserve to divert or hold, but can't use it for fear of some other problem (weight and balance, say).
Only if you absolutely NEVER NEVER will ever have to use that fuel can you consider it to be in the nature of ballast.
(Someone might tell me operationally that's wrong, but from an engineering/design approach, we can't assume that reserve fuel will not be used one day)