Hi Karen,
Check with you school or district re. the webinars. Our district holds the zoom license for everyone, and our admins have access to webinars (it's how they do community meetings and back to school night, for instance). Our admin let us borrow her webinar ability for the shows.
But changing the settings works for everything, it's just much more obvious on webinar. When we rehearsed, we did so on regular zoom and were able to do the layering, though with more cutting out than happened in the webinar. The cutting out is when the echo cancelling programming decides something is background noise versus intended communication. Recently, for instance, we had a student playing "Linus and Lucy" on a piano and the sound of her turning the pages was louder than the music; she hadn't done the switching to original sound or the change in echo cancellation yet.
We discovered that you have to mess around with placement of different sound sources in relation to you mics. Oh, and make sure that everyone has the most up-to-date version of Zoom. But, yeah, it works! And, yes, you absolutely cannot use it for live singing where the music source is at a different location from the singer. We had students play their own music and sing to that, and that worked. It requires quite a bit of moving things around physically until you get the best sound combination, but it worked just fine. And, of course, it only works for solos (unless you've got a group of singers who live in the same house, I guess.)
If you've got any more questions, feel free to ask.