Hey Plutarch, those Richardson Ground Squirrels got me to thinking. As follows...

Since the necessary pre-adaptations involve both sending silent alarms and receiving silent alarms, it seems likely that both abilities were selected and enhanced from a pre-existing wide audible range simultaneously.

What about humans like you and me? Is whispering merely a degraduation of loud talk, or have we too evolved a specialized audible version of the inaudible squirrel screams?

Try this experiment...say "Be quite! Here he comes!" in a loud voice. Then begin lowering your voice in stages until it is almost inaudible. Notice what happens near the lower end of the scale. At one point the continuum of your lowering voice breaks abruptly into a whisper. A whisper which is vastly different from the construct of the spoken word, and a system of communication that uses very different combinations of air flow and larynx, lips, and mouth.

Human kids don't have to go to school to learn to whisper.
The ability to whisper is innate with our species and for many years whispering kept us fed and helped us from being eaten.

( This post, of course, is an aside. I'll return in a moment and address one of your many central points. )