ShallowEarth ? in response to your query:
Controller: VeraPlus
Firmware: 1.7.3831 (latest available)
Accessory Switch Capabilities: 211,156,0,4,18,1,L,R,B,RS,|39,43,44,112,114,115,117,119,133,134,135,
Update: everything was fine for about 3 weeks of normal usage until I decided to stress test my system (all I did was depress the Accessory switch 4-5 times in rapid succession). As you predicted, the ToggleState action is not the solution ? after the stress test, when I depressed the Accessory switch, the indicator light on the Accessory switch toggled to the state opposite the status of the circuit the Master switch was controlling. (I could still toggle the Accessory switch indicator light by depressing the switch again to get it back in sync with the Master switch). So I deleted the ToggleState action from the Master switch.
Good news: SetTarget has miraculously appeared on the list of available services for the Accessory Switch! (It only took 3 weeks - see image.) I added SetTarget = True to two scenes, using the Master switch (on or off) as the trigger for each scene. What I discovered was that SetTarget = True results in the indicator light being turned on. So I edited the Master switch off scene to SetTarget = False. After a day of use, so far so good. In summary, what I have done to get the indicator LED on a Cooper 9517 Accessory switch to remain in sync with a 9501 Master switch:
1. Added Group ID 1 to both Accessory switch and Master switch associations (via Device Options - Add Group), and set the associations to each other
2. Set Parameter 10 using 1 byte hex, and set to 1 (Master switch only)
3. Waited 3 weeks for SetTarget action to become available for the Accessory switch (perhaps depressing it rapidly multiple times is what made the SetTarget action available?)
4. Created a scene triggered when the Master switch is turned on ? and via Advanced Editor Add Action, set the Accessory switch SetTarget to True.
5. Created a scene triggered when the Master switch is turned off ? and via Advanced Editor Add Action, set the Accessory switch SetTarget to False.
Remaining problem: how to instantly sync two Cooper Master 9501 switches. I have scenes that keep them in sync, but there are frequently 2+ second delays (up to 8-10 seconds) in response time. Ideas?