Just out of curiosity, I enabled CEC on the N2, Onkyo (RHID) and Sony TV (Bravia) for a trial. The CoreElec CEC controls are quite detailed, so I was able to have it active but not doing very much. Unfortunately, I still had audio dropouts after a while. And sure enough, mysterious changes to AVR and TV inputs started happening which totally broke my well-sorted Harmony setups. I now have CEC all disabled again.
One thing just occurred to me: I can try listening to some music from my PVR, (also on HDMI and able to output PCM), again with the TV off. Will report back later …
Thanks for your help and support!