Controlling the correlations and electronic reconstruction at the interface of transition metal oxide heterostructures provides a new pathway for tuning their unique physical properties. Here, we investigate the effects of interfacial nonstoichiometry and vertical phase separation on the magnetic properties and proximity-induced magnetism of epitaxial La_0.7Sr_0.3MnO₃ (LSMO)/SrTiO₃(001) oxide heterostructures. We also reinvestigate the recently observed inverse hysteresis behavior reported for this system, which we find emanates from the remanent field of the superconducting solenoid and not from antiferromagnetic intra-layer exchange coupling in low coercivity LSMO thin films. Combined atomically resolved electron energy loss spectroscopy, element-specific X-ray magnetic circular dichroism, and interface-sensitive polarized soft X-ray resonant magnetic reflectivity show the formation of a Mn³⁺-enriched interfacial LSMO layer, of a Ti³⁺-derived magnetic interface layer coupled ferromagnetically to La_0.7Sr_0.3MnO₃, together with a small density of O-vacancies at the interface. These results not only advance the understanding of the magnetism and spin structure of correlated oxide interfaces but also hold promise for practical applications, especially in devices where the performance relies on the control and influence of spin polarization currents by the interfacial spin structure.