Hydrogen is currently being investigated as an energy vector to decarbonize the transport sector, leveraging its numerous advantages as a fuel for internal combustion engines. This study builds on an experimental campaign aimed at assessing the impact of hydrogen enrichment in methane mixtures. Specifically, it evaluates the influence of hydrogen in a fully premixed, passive pre-chamber-equipped, spark-ignition Rapid Compression Expansion Machine (RCEM) under varying equivalence ratios and hydrogen contents. Additionally, the experimental dataset serves to validate the predictive capability of the flamelet-based Extended Coherent Flame Model (ECFM) in RANS numerical simulations. Overall, the model demonstrates solid predictive capabilities, successfully capturing the experimental trends across all mixture compositions. In particular, it accurately predicts stoichiometric cases with hydrogen content ranging from pure hydrogen to a 25% volumetric ratio in methane. However, in lean cases (φ = 0.625), the model exhibits greater discrepancies, requiring more extensive point-to-point calibration of the alpha stretch parameter. These discrepancies, along with the need for point-by-point calibration, are closely linked to the adaptability of the combustion model when the combustion regime is modified, due to variations in the chemical properties of the fuel blend. Furthermore, the pre-chamber system introduces different combustion processes as the flame is propagated from the pre-chamber to the main chamber, where combustion conditions differ significantly. The turbulence-chemistry interaction was analysed to link the calibration process to the combustion regime, using a detailed examination of the Borghi–Peters diagrams. Complementarily, two different kinetic mechanisms were employed to generate laminar flame speed values, enabling an assessment of the reliability of kinetic descriptions in parallel with turbulence–flame interaction modelling, thereby providing a more robust overall analysis.