To close this out for anyone following…
I solved my problem and got everything up and running. The fastest route appears to be to simply NOT use OneSignal.
I abandoned OneSignal in favor of Google’s Firebase Cloud Messaging.
I also used the Firebase Cloud Messaging plugin.
The whole thing was up and running in under 15 minutes. It does exactly what I needed for my particular use case, and it all worked as advertised.
I never did hear anything useful from OneSignal’s support.