We derive the full second-order action for scalar perturbations in the disformal RCD galactic sector including pressureless dust. We prove the kinetic coefficient receives a positive-definite matter correction DeltaQ > 0, yielding c²ₛ = c²ₒ, ₐₒ/ (1+epsilon) with epsilon > 0. The critical density for subluminality is rhocrit ~ rho₂ₑ₈ₓ, ₂₎ₒ₌₎, implying epsilon >> 1 throughout any gravitationally bound structure. In the solar neighborhood, epsilon ~ 3e5 and cₛ/c ~ 0. 002.
Arturo Cerezo (2026) studied this question.