Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. revealed unexpected overdensities of candidate H alpha emitters near the similar to 1-Mpc-wide shock fronts of the massive (similar to 2 x 10(15) M-circle dot) `Sausage' merging cluster, CIZA J2242.8+5301. We used the Keck/Deep Imaging Multi-Object Spectrograph and the William Herschel Telescope/AutoFib2+WYFFOS to confirm 83 H alpha emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster subcores (away from the shock fronts) show high {[}S II] 6716/{[}S II] 6761 and high {[}S II] 6716/H alpha, implying very low electron densities (<30 x lower than all other star-forming galaxies outside the cluster) and/or significant contribution from supernovae, respectively. All cluster star-forming galaxies near the cluster centre show evidence of significant outflows (blueshifted Na D similar to 200-300 km s(-1)), likely driven by supernovae. Strong outflows are also found for the cluster H alpha active galactic nucleus (AGN). H alpha star-forming galaxies in the merging cluster follow the z similar to 0 mass-metallicity relation, showing systematically higher metallicity (similar to 0.15-0.2 dex) than H alpha emitters outside the cluster (projected R > 2.5 Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (similar to 10(15) M-circle dot) clusters can provide the conditions for significant star formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100 Myr.