The Paris Agreement prioritised international bottom-up climate negotiations. Meanwhile, research has asserted the coal exit as a prerequisite for Paris-consistent pathways. The Powering Past Coal Alliance (PPCA), an opt-in initiative toward phasing-out coal-fired electricity by mid-century, embodies both paradigms but currently encompasses just 5% of global coal demand. To assess its long-term prospects against Paris-consistent pathways, we couple the energy-economy model REMIND to an empirical coalition accession model and demonstrate a novel scenario analysis technique, Dynamic Policy Evaluation (DPE). Capturing co-evolutionary feedbacks between policy uptake and global energy markets, we simulate nationally-and-temporally-fragmented PPCA accession and analyse its sensitivity to coalition growth, sectoral ambition, and Covid-19-related uncertainty. Surprisingly, we find that virtually-global PPCA participation achieves <3% of 1.5oC-consistent coal declines, as non-electric consumption remains unregulated. In contrast, our median-estimate scenario (82% accession) assuming economy-wide coverage achieves ~53% efficacy (virtually-global: ~85%), suggesting that the PPCA should prioritise policy ambition over coalition expansion.