Zero Fuel Weight:
MEW - manufacturers empty Weight
OEW - Operators empty weight
OEW + Payload is a function - the delta you looking for is the max mass allowable, ipso Max ZFW (fuel that can be up loaded)
MEW is the A/C mass (static) as produced by OEM
OEW is MEW + customer add ons (customization)
OEW changes based on customer requirements - not a military problem as slinging a few missiles on an fast jet is not the problem that confronts an Ops manager for a civil operation
OEW - ZFW = total fuel load max allowable based on A/c allowables
structure moments and bending are more to do with repairs and static nominal zero for stress jacking ( it's more complicated than that but I can't be bothered going into it)
So, in summary - ZFW is the derived sum of available mass after all optional operational considerations are allowed for i.e how much fuel can I upload for this operational sector is one option, but in general mass and loading are derivatives of commercial Ops