Zero-dimensional cesium lead halide (Cs4PbBr6) perovskites are distinguished by their structural robustness and defect-mediated optical properties, yet the origin of their emissive behavior remains elusive. Here, we reveal a hierarchical defect architecture in Cs4PbBr6 microcrystals, wherein embedded CsPbBr3 inclusions hosting bromine vacancies (VBr), form multilevel CsPbBr3-VBr coupling states that govern charge-carrier dynamics. A micro-solvent–assisted mechanochemical strategy employing trace DMSO solvent enabled precise defect modulation while achieving low process mass intensity (PMI ∼5, far below conventional solution routes, ∼180) and structural fidelity. Comparative analyses of dry-synthesized (D-Cs4PbBr6), DMSO-assisted (B-Cs4PbBr6), and ligand-mediated (L-Cs4PbBr6) microcrystals reveals the presence of hierarchical defect states are confirmed by low-temperature photoluminescence, cathodoluminescence, and transient absorption spectroscopy, with DFT calculations identifying mid-gap states (∼1.93 eV) responsible for NIR-range emission. While shallow states drive this emission, detrimental deep traps are effectively neutralized by sulfate-ion passivation, extending carrier lifetimes from 35.48 to 41.43 ns and enabling suppression in photoluminescence quenching. The optimized bromine-vacancy densities facilitate enhanced CO2 photoreduction (70.21 µmol g−1 h−1, apparent quantum yield 1.42%), nearly 1.6× higher than defect-free analogues, consistent with DFT-predicted shallow VBr states enhancing charge availability. These findings establish a scalable, defect-engineered mechanochemical route for multifunctional perovskites, uniting robust luminescence and efficient photocatalytic energy conversion.