Linear growth rate as a function of the wavenumber k. The black curves show the two asymptotic limits: the uncoupled case (No coupling) and the perfectly coupled case (Perfect coupling). The blue curve corresponds to the formulation with ξ = 1 used in Paper I, while the red curve shows the physically consistent case ξ = d, where the collision frequencies scale with the density contrast across the interface (constant drag coefficient α). The dashed red curve labelled ‘Equivalent with ξ = d’ shows the ξ = 1 formulation evaluated with an effective collision frequency ν_nc,eq chosen so as to reproduce the ξ = d dispersion curve over the intermediate-k range.