Fibre-Level Admissibility from Conjugate Weil Blocks: Structural Derivation of the Pair Observable

O16 established that the physically relevant observable for the Weil-block capacity problem is defined on conjugate pairs \c, q-c\ rather than on individual spectral blocks, yielding the doubling ₏₀₈ₑ = 2\, c 7. 44 consistent with the phenomenological range 7. 4, 10. 6. However, the identification of conjugate pairs with the fibres of the non-injective projection~ was stated there as a structurally motivated hypothesis rather than a derived result. The present paper provides the structural derivation. We show that the raw Gram-Schmidt redundancy counts are exactly identical when the initial supports coincide, and are structurally equivalent in general, establishing that conjugate blocks carry the same dynamical information. The proportionality constant r (c, q) appearing in O16 is shown to be a normalisation artefact of the pipeline, not an intrinsic invariant of the representation. The only structurally robust quantity is therefore the equality c = ₐ-₂, which follows from the conjugation identity ₐ-₂ = c and the norm-invariance of Gram-Schmidt orthogonalisation. This closes the logical gap left open in O16 and requalifies the O12-O15 results as correct analyses of a block-level observable that is not the appropriate physical unit. A key outcome of this work is the elimination of a misleading structural hypothesis: the factor r (c, q), initially suspected in O16 to carry arithmetic significance (quadratic Gauss sums or character multiplicities), is shown here to be purely normalisation-dependent and gauge-like. This simplifies the theoretical structure by removing an apparent invariant that was not one.