Rechargeable magnesium–sulfur (Mg–S) and sodium–sulfur (Na–S) batteries are widely regarded as promising next-generation energy-storage systems due to their high theoretical energy density, intrinsic safety, and low material cost. However, their development remains fundamentally challenged by sluggish multivalent ion transport, unstable electrolyte–electrode interfaces, and severe polysulfide shuttling. In this work, we propose and explore the use of phosphoric acid (H3PO4) as an unconventional electrolyte modifier to regulate Mg2+ solvation behavior, ionic conductivity, and solid–electrolyte interphase (SEI) evolution in Mg–S cells, rather than as a finalized optimization strategy. In parallel, a multifunctional sulfur cathode composed of graphene nanoplatelets, silicon carbide, and barium titanate is designed as a model platform to examine combined electronic conductivity, polysulfide confinement, and mechanical stabilization effects. Electrochemical and structural analyses (EIS, CV, SEM, and EDS) are employed to assess how H3PO4-induced solvation and interfacial changes influence Mg2+ diffusion and charge-transfer processes, revealing both performance gains and inherent limitations. For comparison, the same sulfur composite is evaluated in Na–S cells using a NaPF6-based electrolyte, where fast initial Na+ transport is observed but rapid capacity fading persists due to unresolved polysulfide shuttling. Rather than demonstrating performance optimization, this study highlights the opportunities and constraints of solvation-chemistry engineering combined with advanced sulfur-cathode design, offering critical insights to guide future development of halogen-free, safe sulfur-based battery systems.