Large unmanned cargo airships may support heavy-lift logistics in regions without runway infrastructure, but payload release produces a rapid buoyancy surplus and changes the vehicle mass properties. This study develops a simulation framework coupling six-degree-of-freedom variable-property flight dynamics, an active seawater ballast system, and constrained ballast-flow allocation. The dynamics are referenced to a fixed body origin and retain the spatial-mass-matrix derivative and declared exchange-momentum wrench. A one-dimensional Method-of-Characteristics (MOC) solution provides a numerical reference for the reduced line-inertance runtime model. Fitting yields $L_{\mathrm{eff}}=55.240$ m and a 15.3% closure-interval normalized root-mean-square error (NRMSE), providing cross-model verification rather than experimental validation. Under the nominal 1201 s mission, the variable-property-aware case satisfies the predeclared criteria with a final-altitude error of 2.985 m and a steady-climb pitch RMS error of ${0.165}^{\circ }$. A fair frozen-inertia ablation also passes and produces slightly lower nominal errors (2.806 m and ${0.157}^{\circ }$); hence, nominal superiority is not claimed. Across the five-point pitch-inertia sweep (0.70 to 1.30 times nominal), the variable-property-aware controller keeps the steady-climb pitch RMS error within ${0.137}^{\circ }$–${0.165}^{\circ }$, whereas the frozen-inertia proportional–integral–derivative (PID) controller spans ${0.153}^{\circ }$–${0.230}^{\circ }$; at 1.30 times nominal inertia, the variable-property-aware controller reduces pitch RMS error by 31.8% and settles 12.9 s earlier. All 21 independently rerun cases that jointly scale the nominal pump and valve time constants from 1.0 to 3.0 satisfy the declared criteria. In the $N=20$, $\pm 5\%$ local parameter-dispersion study, all plotted trajectories remain state-bounded, while the wide altitude spread precludes a uniform tracking or reliability claim. The evidence supports numerical feasibility within the explicitly tested nominal, inertia, and actuator-time-constant cases while requiring configuration-specific trim and controller rematching before extrapolation; it does not constitute a reliability probability or global stability proof.