INDEPENDENT SAVES. Confusingly, there are a couple other options and configurations that work outside of the process above. This is because they are governed by their own configuration files, and not the typical retroarch.cfg or override cfg/rmp files mentioned above.
By mistake I configured the keys in the MAME 2003 core completely wild, so that now nothing fits anymore. (Important: directly in the emulator, not in the retroarch interface).
Uninstalling and reinstalling the core did nothing. Is there a tip how I can reset the Core internal settings?