You're doing everything wrong.
You want to go from MP3 to WAV and do everything there, then mix it in with anything else that you need to mix with and reduce the volume then. Not before. Not after.
If you go MP3 > WAV > MP3 > mix > MP3 you will have audio degradation that you can avoid.
The fewer steps, the better, unless you are purely dealing in a lossless format like WAV or FLAC. I wouldn't recommend ALAC or RIFF.
But, then again, you probably don't need the best audio quality you can get either. Good enough is enough a lot of the time.