We assemble a large comprehensive sample of 2534 z similar to 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 30-50, 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 similar to 2-9 galaxies, finding sources as faint as -12.4 mag at z similar to 3 and -12.9 mag at z similar to 7. We explicitly demonstrate the power of the surface density-magnification relations sigma(z) versus mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the sigma(z) versus 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) versus mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z similar to 7 to z similar to 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.