WASAPI Exclusive mode does not lock volume.
You can set volume to disabled, to get that effect.
Did you check the other setting I mentioned, Maximize device volume during playback? Check it. And you can try disabling volume, and check the other volume settings in MC.
If audio path is empty as you say, and you disable volume and set the option I mentioned earlier, MC will output without attenuation. If your other app then still sounds louder, it may be doing something to boost its volume, or your sound driver settings might need adjustment.