SEBI's 2014 mandate requiring at least one woman director on the board of every listed company — subsequently strengthened to two independent women directors for top 500 listed entities by market capitalisation (SEBI circular 2018) — created a quasi-experimental setting for examining the causal effects of board gender diversity on firm performance and leadership pipeline development in India. The pre-mandate baseline (2013 census: 4.8% women on BSE 500 boards) and the post-mandate trajectory (2024: 19.6% women on boards) enable regression discontinuity design analysis of the mandate's effects that avoids the self-selection bias afflicting voluntary diversity studies. This study analyses a panel of 486 BSE-listed companies from 2012-2024, examining the causal effect of board gender diversity on Tobin's Q, ROA, ROE, and sustainability reporting quality, using regression discontinuity design at the mandate implementation threshold and instrumental variable estimation using the SEBI mandate as an instrument for board diversity. The study also examines the pipeline paradox — whether mandatory board appointments have accelerated or been disconnected from the development of women in senior management positions — through analysis of C-suite gender data from annual reports. The findings confirm positive diversity-performance effects (Tobin's Q premium of 0.24 for highest diversity quartile versus lowest), but reveal a persistent board-executive gap (board women: 19.6% versus C-suite women: 12.4%) and significant sector heterogeneity in both representation and performance impact.