Two-dimensional atomic arrays provide versatile free-space quantum optical interfaces by coupling photons to collective lattice modes. Here we show that the Rayleigh–Wood anomaly provides a sharp control mechanism for both linear and nonlinear photon scattering from a single atomic layer. As the lattice spacing crosses the diffraction threshold, newly opened radiative channels rapidly broaden the dominant collective modes, converting specular reflection into diffuse off-axis scattering. In transmission, momentum-space Fano interference between the incident and collectively scattered fields produces single-photon intensity zeros that strongly reshape the momentum-space correlation pattern. An integrated nonlinear contrast reveals a pronounced channel asymmetry: upon opening the Rayleigh channels, the connected two-photon contribution drops by several orders of magnitude relative to the factorized background in transmission, while remaining close to unity in reflection. These results establish radiative diffraction thresholds as a control principle for quantum nonlinear optics in single-layer atomic arrays.