The ambient processing of perovskite solar cells (PSCs) is crucial for scalable industrialization; however, moisture-induced degradation and non-uniform crystallization remain significant hurdles. In this work, we demonstrate a facile, ambient-processable two-step intercalation strategy that premixes cesium iodide (CsI) directly into the PbI2 precursor to regulate the crystallization and carrier dynamics of methylammonium lead iodide (MAPbI3) PSCs. By bypassing conventional solution-processed solubility limits, this pre-intercalation approach ensures a highly uniform Cs+ distribution. Mechanistically, this kinetically promotes Ostwald ripening during crystal growth and thermodynamically relieves native lattice microstrain. Consequently, the optimal CsI concentration (2 mg mL-1) significantly enlarged the perovskite grain size (from 161 nm to 255 nm), reduced dislocation density, and suppressed deep-level trap states. These structural refinements effectively mitigated trap-assisted recombination and improved charge extraction. The optimal ambient-processed device achieved a champion power conversion efficiency (PCE) of 23.17% (average 20.57 ± 1.28%) with minimized hysteresis. Furthermore, applying this CsI-intercalated architecture in a strictly controlled environment (glovebox) validated its fundamental optoelectronic potential, yielding a true PCE of 23.78% with an excellent fill factor of 80.9%. Finally, the unencapsulated devices demonstrated remarkable durability, retaining 97.7% of their initial efficiency under continuous operational illumination and 82% after 30 days of ambient storage. These findings establish controlled CsI precursor intercalation as a scalable and mechanistically sound pathway for highly efficient and stable perovskite photovoltaics.