This paper analyzes fundamental physical constraints on magnon-based quantum memory systems. We examine collective-mode behavior and magnetohydrodynamic (MHD) limits that govern the scalability of photon-magnon quantum state transfer. Key findings: - Spin wave propagation limits spatial addressing resolution - Magnetic Reynolds number R_m ≈ 10⁻¹² confirms diffusion-dominated regime - Maximum achievable channels: N_max ~ 10³-10⁴ via frequency-based addressing - Channel crosstalk analysis provides design guidelines This work complements the theoretical framework (Paper I) and experimental proposal (Paper II) in a three-paper series on magnetic quantum memory. Keywords: quantum memory, magnons, spin waves, magnetohydrodynamics, photon-magnon coupling