  Models of solar-like oscillators yield acoustic modes at different
frequencies than would be seen in actual stars possessing identical interior
structure, due to modelling error near the surface. This asteroseismic "surface
term" must be corrected when mode frequencies are used to infer stellar
structure. Subgiants exhibit oscillations of mixed acoustic ($p$-mode) and
gravity ($g$-mode) character, which defy description by the traditional
$p$-mode asymptotic relation. Since nonparametric diagnostics of the surface
term rely on this description, they cannot be applied to subgiants directly. In
Paper I, we generalised such nonparametric methods to mixed modes, and showed
that traditional surface-term corrections only account for mixed-mode coupling
to, at best, first order in a perturbative expansion. Here, we apply those
results, modelling subgiants using asteroseismic data. We demonstrate that, for
grid-based inference of subgiant properties using individual mode frequencies,
neglecting higher-order effects of mode coupling in the surface term results in
significant systematic differences in the inferred stellar masses, and
measurable systematics in other fundamental properties. While these systematics
are smaller than those resulting from other choices of model construction, they
persist for both parametric and nonparametric formulations of the surface term.
This suggests that mode coupling should be fully accounted for when correcting
for the surface term in seismic modelling with mixed modes, irrespective of the
choice of correction used. The inferred properties of subgiants, in particular
masses and ages, also depend on the choice of surface-term correction, in a
different manner from both main-sequence and red giant stars.
