Selective etching has emerged as a key method for synthesizing 2D materials, with the conversion of MAX phases to MXenes being by far the most widely studied and reported example. While traditional methods rely on etching in primarily acidic aqueous media, molten salts offer an intriguing alternative. However, the current understanding of MAX phase reactivity in molten salts is limited, restricting our ability to predict reaction outcomes. In this study, we present a computational framework that uses process-specific phase diagrams to model A-element substitution and MXene formation, as well as competing side reactions. Applying this approach to Ti3AlC2, V2AlC, and Ti2AlN in ZnCl2 molten salt, we reveal distinct reaction behaviors despite identical redox potentials-defined here by the Al-to-Zn exchange-of key processes. Our findings underscore the limitations of predicting reactions based solely on redox potentials and show that our model can capture key trends in MXene synthesis. Beyond MXenes, our methodology lays the groundwork for identifying new 2D materials accessible through molten salt etching.