In the field of Business Administration, Management and Corporate Governance, the shift to social commerce platforms has altered consumer purchasing pathways and brand communication effectiveness. This empirical investigation systematically examines Source Credibility and Influencer Authenticity in Social Commerce: Impact on Millennial Brand Attachment through a multi-stage experimental methodology and rigorous quantitative analytical framework. Utilizing cross-sectional survey of 620 social media consumers and structural equation modeling grounded in S-O-R framework, data were gathered across multiple operational cycles and validated against established international benchmarks. The statistical and computational results reveal that perceived influencer authenticity directly drove emotional brand attachment, which fully mediated repeat purchase intentions and brand advocacy. Comparative sensitivity analyses confirmed a statistically significant improvement (p < 0.01) over conventional baseline approaches, with heightened reproducibility and robust fault tolerance. These comprehensive findings provide actionable theoretical insights and practical implementation guidelines for digital marketing strategists and direct-to-consumer brand managers. Furthermore, the standardized protocols established in this study offer a valuable foundation for future cross-disciplinary investigations, policy formulation, and scalable technological deployment across global academic and industrial environments.