Mass Inflation Halted by Modulus Field Back-Reaction in CCEGA Regular Black Holes

The companion paper established that the inner (Cauchy) horizon of CCEGA regular black holes carries a negative surface gravity κ₁ ≈ −3.47 c³/GM, driving the mass inflation instability, but that the modulus field φ damps the effective growth rate by a factor G_eff(r₁)/G ≈ 0.23 in the linear regime. Here we extend the analysis to the full non-linear regime using a coupled perturbation-field system. We find that the φ field back-reaction completely halts mass inflation: the growing perturbation drives φ → 1 at the inner horizon, extinguishing the effective gravitational coupling G_eff → 0 and terminating the instability on a finite timescale v_stab ≈ 11.5 GM/c³ (≈ 1.7 ms for 30 M☉). The critical back-reaction coupling required for stabilisation is λ_crit ≈ 2×10⁻⁶, while the physical CCEGA coupling is λ_phys ≈ 0.23 — a safety margin of ~10⁵. The inner horizon is therefore non-linearly stable in CCEGA for all physically motivated parameter values. This result closes the primary open problem of the CCEGA information preservation programme: the regular core is permanent, the Dirichlet boundary condition is robust, and the unitarity of the scattering matrix is confirmed against non-linear perturbations.