In previous work, we have characterized the ``Universe'' as a maximal, consistent, and complete ontological object \mathfrak U within a given foundational category, providing unified time scale, boundary time geometry, causal--entropic--observer axioms, and unified encoding of ``detail data'' \mathcal D=([E],\{f_\alpha\}). This system achieves unification at structural and parametric levels but retains a critical gap: physical laws themselves still appear in an intrinsic, fragmented manner, e.g., Einstein equations, Yang--Mills equations, Dirac equations, Navier--Stokes equations, multi-agent resource allocation dynamics, etc., have not yet been demonstrated as unique consequences of a single ontological principle. The goal of this paper is to bridge this gap: within the framework of the Universe Ontological Object \mathfrak U, unified time scale, and boundary time geometry, we construct a single consistent variational principle equation \delta\mathcal I[\mathfrak U]=0, equation and prove: 1. This variational principle consists only of three universal requirements: (i) Causal--Scattering Consistency (unitarity and macroscopic causality); (ii) Generalized Entropy Monotonicity and Stability (``Generalized Second Law'' under unified time scale); (iii) Observer--Consensus Consistency (all local observers' models--readouts must be embeddable into the same \mathfrak U). 2. On small causal diamonds, variation of geometry and states derives Einstein equations G_{ab}+\Lambda g_{ab}=8\pi G\langle T_{ab}\rangle and energy--momentum conservation from this principle; 3. Under fixed geometry and unified scale, variation of boundary channel bundles and field content derives local gauge invariance and Yang--Mills field equations; 4. On given geometry and gauge structure, variation of matter fields and scattering data derives Dirac/Klein--Gordon field equations and local quantum field theory (satisfying microcausality and spectral conditions); 5. In long-wavelength and coarse-grained limits, variation of resolution connections and entropy functionals derives generalized hydrodynamics and effective Navier--Stokes equations, as well as entropy gradient flow dynamics for multi-agent resource allocation. Thus, general relativity, gauge field theory, quantum field theory, fluid and statistical physics, and multi-agent dynamics are all shown to be necessary conditions of the same universe consistent variational principle at different resolution levels and boundary conditions. This strictly completes physical unification: there are no longer mutually independent ``force laws'' or ``matter equations'', but only one Universe Ontological Object \mathfrak U and one consistent variational principle. Specific theories are merely effective unfoldings of this principle in different limits. The appendices provide specific construction of this consistency functional \mathcal I[\mathfrak U], variational derivation on small causal diamonds, unified treatment of gauge structure and field content, and outlines of reconstruction proofs for local quantum field theory and hydrodynamic limits.