The growing demand for spatially efficient multi-beam microwave systems in satellite communications, 5G and 6G terrestrial networks, and fixed wireless backhaul has renewed focus on passive reflector arrays as high-gain, low-loss beam-forming structures. While Physical Optics (PO) remains the standard analytical method for electrically large reflectors, its failure to model edge-diffraction effects at finite aperture rims introduces quantifiable inaccuracies in multi-beam configurations. This paper presents a hybrid PO and Physical Theory of Diffraction (PTD) modeling framework implemented in MATLAB for the design and parametric analysis of passive reflector arrays supporting simultaneous multi-beam operation. Using a design geometry of aperture diameter D = 1.0 m, focal length f = 0.4 m, and UTD wedge exponent n = 2.0, the framework quantifies the impact of edge diffraction on sidelobe level, beam isolation, and scan performance. Results demonstrate that PTD correction elevates the far-sidelobe floor by up to 6 dB and reduces achievable beam isolation by up to 2.2 dB relative to PO-only analysis. The 20 dB isolation threshold for frequency-reuse satellite systems requires 0.3 beamwidths more separation under PTD-corrected analysis than PO-only prediction. A comprehensive parametric sensitivity analysis identifies inter-element spacing as the dominant isolation control variable with a sensitivity index of 8.2, feed taper as the primary sidelobe control parameter (7.2), and aperture diameter as the primary gain determinant (9.2). These findings establish PTD integration as essential for accurate multi-beam passive reflector design and provide quantitative design guidelines for system-level multi-beam planning.