Sad to say that I was unable to find a solution. I have disabled HDMI-CEC and use the BT-500 TV and STB power buttons to separately power the TV and PS6 on/off. I either use my Yamaha remote or my feet (walk over to my Yamaha Receiver) to control my receiver. A very unsatisfactory situation given that my Firestick remote controlled everything perfectly for 10% of the price of a PS6!
That said, I really like my PS6s (I have three), they work perfectly on my other two TVs that just have sound bars attached rather than the whole home theatre set-up. I also really like my Yamaha receiver and can't justify replacing it in the hope a new receiver will play nice with my PS6. Because the current situation works but is less than satisfactory, eventually I will either replace the Yamaha, a PS6 update will fix the problem or someone else will figure out a solution.
I should add that Yamaha Canada support was incredibly responsive and helpful but hold the position that their device is working as it should, as proven by how it works so well with other devices, e.g. the Firestick and all other devices in my home theatre.