Max Zero fuel is 12,324lbs Empty = 8,243lbs (Approximate depending on specific aircraft) so the max payload (including the 2 crews for this computation) is = 4,081lbs.
True - but MZFW is a structural limitation (how much weight/stress the wing bolts can handle) rather than an aerodynamic limitation (how much weight the wings and engines can lift). In this case, the wings didn't come off. Instead the airplane apparently could not maintain altitude with loss of an engine.
I'd agree that on a short flight requiring substantially less than full fuel, but with a large payload, one should usually hit the MZFW limit before reaching MTOW.