And that is exactly what I want, but can’t achieve.
As I mentioned Signia says that is what the streamline mic should do, but doesn’t.
Signia support also says to set the device (laptop/phone) audio to mono (yes mono summing as you stated, but everything simply calls it mono).
e.g. Windows 10:

Signia tech support said in an email that the streamline mic should automatically combine the left and right channel as a single channel sent to a single hearing aid without having to set the devices to output mono (combine).
I am hoping I can get the software and try to achieve what my audiologist couldn’t get working.
If I am successful, I can let her know what the solution is. I am sure I can spend a lot more time trying to get this working than my audiologist could (for obvious reasons).
I am already well aware of the difference between true stereo and true mono. I know that mono is only a single channel, whereas stereo is left and right. I am only repeating what Signia tech support told my audiologist and me.