We report a scalable hydrogen-free synthesis of heterophase 2H/1T′ WSe₂-based gas sensor for ultrasensitive room temperature NO₂ detection. Moving beyond conventional defect engineering, this work leverages deliberate phase and morphological synergies to overcome traditional kinetic trade-offs in 2D material-based sensors. Unlike conventional 3D structures, this open hierarchical nano-butterfly-like morphology prevents van der Waals restacking and maximizes exposed active edge sites for rapid gas diffusion. Quantitative phase engineering yielding an optimized surface of kinetically trapped 1T′ domains (∼17%) within a dominant 2H matrix (∼83%) uniquely couples the semiconducting baseline stability of the 2H phase with the rapid, metallic charge–transfer channels of the 1T′ twin boundaries. This synergistic transduction mechanism demonstrated a remarkable 40% response toward 800 ppb NO₂, a theoretical limit of detection (LOD) of 4 ppb, and robust environmental stability over 40 weeks without requiring external thermal or optical activation. Furthermore, density functional theory (DFT) simulations accelerated by Bayesian optimization successfully validated the experimental findings, identifying physisorption as the dominant interaction mode for NO₂ on WSe₂ and providing insight into different adsorption modes on 2H and 1T′ domains. These results establish a highly quantitative, structurally engineered framework for next-generation low-power transition metal dichalcogenides (TMDs) based gas sensors.