The Etherington relation, D_L = (1+z)²D_A, holds in any metric theory with photon conservation, so its violation would be a fire alarm. The tests, supernovae against BAO, clusters, and lenses, return a chronic scatter of 1-to-2σ departures whose sign varies with the D_A source, never confirming, never clearing, entangled with hydrostatic bias on one side and the sound-horizon calibration on the other.
The standard tests assume both distance families measure one homogeneous universe: any eta(z) departure must then be new photon physics or broken gravity, since the model permits no third reading. The possibility that D_L and D_A datasets legitimately sample different volumes of a structured expansion is excluded before the data are compared.
SCT defends the relation and indicts the comparison. The duality itself holds: SCT's gravity is metric, GR with three structural modifications that touch sources, not photon propagation (P65), photons are conserved, and geodesics are null, so genuine Etherington violation would falsify SCT alongside everything else. The persistent apparent departures are the gradient's cross-term: supernova D_L samples are anchored in the enhanced local volume and weighted by their host geography, while BAO and cluster D_A samples weight the broader field and carry their own calibration chains (P17, P19), so eta(z) fit between mismatched samplings of an inhomogeneous Λ_eff field will scatter at the few-percent level with source-dependent signs, which is the published record exactly. The frame-tree layer (P10) adds its registered 10⁻⁴-class redshift offsets, a small systematic eta absorbs.
The prediction is the matched-sightline test: duality checked between D_L and D_A measurements along the same sightlines through classified environments should tighten toward exact validity, while unmatched whole-catalog comparisons keep their scatter, the signature separating a bent relation from a structured universe. Foundations in Paper 1 (P17, P19) with the GR-preserving architecture of Paper 12, From Chaos To Comoving Coordinates (P65).
The premise bill is zero new: the same field that organizes the H₀ camps predicts precisely this pattern of phantom duality violations.
Both readings face execution: a confirmed sub-percent Etherington violation in matched-sightline data, surviving environment stratification, would falsify SCT's metric foundations as thoroughly as ΛCDM's. And the mismatch reading dies specifically if the scatter persists undiminished in environment-matched comparisons, or vanishes entirely in unmatched ones, either outcome breaking the field's cross-term account. Rubin-against-DESI at sub-percent eta runs the test this decade.