  We assemble a large comprehensive sample of 2534 z~2, 3, 4, 5, 6, 7, 8, and 9
galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF)
program. Making use of the availability of multiple independent magnification
models for each of the HFF clusters and alternatively treating one of the
models as the "truth," we show that the median magnification factors from the
v4 parametric models are typically reliable to values of 30 to 50, and in one
case to 100. Using the median magnification factor from the latest v4 models,
we estimate the UV luminosities of the 2534 lensed z~2-9 galaxies, finding
sources as faint as -12.4 mag at z~3 and -12.9 mag at z~7. We explicitly
demonstrate the power of the surface density-magnification relations Sigma(z)
vs. mu in the HFF clusters to constrain both distant galaxy properties and
cluster lensing properties. Based on the Sigma(z) vs. mu relations, we show
that the median magnification estimates from existing public models must be
reliable predictors of the true magnification mu to mu<15 (95% confidence). We
also use the observed Sigma(z) vs. mu relations to derive constraints on the
evolution of the luminosity function faint-end slope from z~7 to z~2, showing
that faint-end slope results can be consistent with blank-field studies if, and
only if, the selection efficiency shows no strong dependence on the
magnification factor mu. This can only be the case if very low luminosity
galaxies are very small, being unresolved in deep lensing probes.
