Along the lines of the discussion on the list...
All I did was install a portable copy of NVDA 2019.2.1, and, install the speechPlayerInEspeak add-on under it.
Then, under that portable copy installation, I browsed to the following relative path:
From there, I copied the file called edward over to the following path, which falls under my actual installed copy of NVDA 2019.3 rc1:
C:\Program Files (x86)\NVDA\synthDrivers\espeak-ng-data\voices\!v
It did prompt me to continue with administrator privileges when trying to copy the file under program files, but, that's standard.
Anyway, when I now select the eSpeak-NG synthesizer, I can now select the edward variant from the list.
Now, I still myself prefer the normal eSpeak-NG gene variant, but, HTH
"...resistance is futile...but, acceptance is versatile..."