Flaps One
You do get a gross weight credit when fuel is put in the wings. The reduction in wing root bending moment due to wing fuel is accounted for when MTOW is calculated. That's one of the reasons center section fuel is burned prior to burning wing fuel.
MZFW is the weight at which all structural regulations regarding flight load factors i.e. gust loads, maneuver loads etc. can be met. The airplane still must pass the regs. with empty tanks. Remember your 2.5g airplane can have more maneuverability than that with full wing fuel and enough airspeed, provided other bits like the h-tail and the engine mounts are good for it.