Closing the loop on this. It’s now fixed. The issue was I had the Vero, AVR, and TV all set to turn each other on/off. I assumed (wrongly) such settings would not interfere with one another. Now I have the Vero set to turn on/off the TV and the TV set to turn on/off the AVR and everything works. Volume control through the Vero remote works again as well.
Thank you! Your information was the key to solving this. I’m guessing devices were getting conflicting (or multiple) on/off commands from various devices and stupidly deciding to ignore all of them. HDMI CEC is as annoyingly half-baked as ever, but that’s no fault of OSMC/Vero. Indeed, the Vero was the only part of this equation that acted as expected.