If you change emulator for a platform you have to tell the games, that default checkbox is just for imports and as you already imported it wont change. Just right click the game and go to edit, select the emulator tab and use the dropdown to switch the emulator. As for retroarch you have selected the mednafen psx core, is that the on you are using? There are two, one standard and another called mednafen_psx_hw, so make sure you have the correct one selected