If you set the property_key: '9' to a percentage, the sound will play. If it’s too long for your liking, you can create a script that sets the volume to 100 percent, waits 5 seconds, sets the volume to 0 percent. As for the other property keys for the “generic event sound notifications”, you will probably have to ask Ring’s engineers on how those work exactly.
We don’t really know. Maybe there isn’t one. The documentation is very sparse. You could try asking Ring (they probably don’t want people to use their devices outside of their own ecosystem, though).
Indicators are standardized by the Z-Wave specification. If you look at the Device Diagnostic data (download from the Device page), it should list which indicator values and property IDs are supported.
property 98 is “Generic event sound notification 3”, which doesn’t tell you too much. But property key 9 is volume level, and 7 is timeout in seconds.
Yes, but the keypad as implemented does seem to follow the specification. Property key 9 is not supposed to switch on the indicator according to the specs, yet it does. The property key 2 is supposed to turn the indicator on and off, but does not even exist.