  This is the second in a series of papers in which we use JWST MIRI multiband
imaging to measure the warm dust emission in a sample of 31 multiply imaged
quasars, to be used as a probe of the particle nature of dark matter. We
present measurements of the relative magnifications of the strongly lensed warm
dust emission in a sample of 9 systems. The warm dust region is compact and
sensitive to perturbations by populations of halos down to masses $\sim 10^6$
M$_{\odot}$. Using these warm dust flux-ratio measurements in combination with
5 previous narrow-line flux-ratio measurements, we constrain the halo mass
function. In our model, we allow for complex deflector macromodels with
flexible third and fourth-order multipole deviations from ellipticity, and we
introduce an improved model of the tidal evolution of subhalos. We constrain a
WDM model and find an upper limit on the half-mode mass of $10^{7.6} M_\odot$
at posterior odds of 10:1. This corresponds to a lower limit on a thermally
produced dark matter particle mass of 6.1 keV. This is the strongest
gravitational lensing constraint to date, and comparable to those from
independent probes such as the Ly$\alpha$ forest and Milky Way satellite
galaxies.
