Linked Complement-Reversed Interface Class and B5-Representation-Invariant Threshold Coalescence on Q5

What happens at the precise threshold where two coupled quantum channels are about to split, and what symmetry structure governs that moment? This paper answers that question within the five-dimensional hypercube Q5 framework. Working from the complement-paired bulk-mediated architecture established in Theorems 12 and 15, we study the linked interface between two fibres and prove three things. First, the threshold behaviour is controlled by a single basis-independent discriminant D = |κ|² − ((ω_A − ω_B)/2)², and this discriminant defines a symmetry class that is invariant under the full architecture-preserving subgroup G_AB of the B5 symmetry group. The threshold is a structural invariant, not a coordinate artifact. Second, eliminating the dressed sector via Schur complement generates no protected-channel transport bridge between the fibres at any order. The interface carries a threshold class without carrying a transport operator: a clean structural separation. Third, at the generic nontrivial threshold, the dressed helicity block undergoes non-semisimple coalescence. An explicit Jordan chain is constructed, showing that the two eigenvalues collide in a way that cannot be diagonalized: a signature of genuine structural degeneracy. Together, these results characterize the linked complement-reversed interface as a mathematically precise object: a G_AB-invariant threshold class realized by Jordan coalescence, in the absence of any protected transport channel. The open boundary (deriving the admissible coupling form from Q5 first principles) is clearly stated and does not affect the theorem. Dependencies: Theorems 7, 12, 14, 15.