This work presents SPT v1.3 — Part VIII, applying the Structural Flux Paradigm of Structural Plenitude Theory to the interstellar object 3I/ATLAS. Building upon the thermodynamic and radiative framework established in Parts I–VI, this article examines how substrate-mediated energy transfer, structural impedance, and coupling efficiency govern the observed non-gravitational behavior, spectral anomalies, and radio signatures associated with 3I/ATLAS. The analysis treats the object not as an anomalous cometary body, but as a structurally responsive system interacting with the viscoelastic vacuum substrate (Pleroma). Observational baselines, residual radiative dissipation, nickel-related spectral features, and predicted dynamical deviations are integrated into a unified, falsifiable framework. This document is part of a frozen multi-part SPT v1.3 release. Each part receives an independent DOI to ensure traceability, version integrity, and resistance to post hoc reinterpretation. No new ontological postulates are introduced; the work strictly applies previously established principles to an interstellar test case.