We can agree that there is a big difference between "perceived by no one" and "perceived by no one but me," right?
Madonna tries out a riff in her head. If it is not so good, then just as well no one else hears it. If it is good, then it easy enough for her to arrange that others hear it. Money in the bank, handy faculty, lucky Madonna. Evolution by natural selection triumphs again.
She couldn't do it if she couldn't do everything with her IV that she could do with real voice. And if she decides to punch it up with a little wow and flutter, then she will think of wow and flutter, and hear those effects as well.
As to the other, I can't breathe in through my mouth and out through my nose. The air flows at all only because the pressure in my lungs is higher or lower than the air around me. The pressure difference is the same, regardless of path. The tide only flows one way at a time.
BTW, In the experiment I mentioned, mere information won't disrupt the process. People speak perfectly well when listening to music, when other people are talking, etc.
It is the specific information that is disruptive in the demonstration, plausible but false feedback about the sound you are producing. You can speak just fine without any auditory feedback (deaf people do that all the time), what is disruptive is false feedback.
And it disrupts because you are comparing it to what you expected to hear... which can also be represented to yourself as a voice in your head, and can function counterfactually (what I would hear if I were speaking, but I am not).
It is notoriously difficult to discuss interior private mental states, so perhaps this is some of what is getting in our way here.