@Nick1234 in the latest 1.4.2 firmware and deco app you are able to see if connections are wired or wireless

@Nick1234 After a bit of playing, I was able to answer a couple of my own questions...

Yes, powerline backhaul works, but only if I set the operating mode to 'Access Point'.  Unfortunately this causes me to lose some of the cool features like parental control, but I guess I can work without it for the time being. 

However, I can't tell whether the units switch to ethernet/powerline backhaul based on speed... or whether it just happens automatically no matter.

Hi @Nick1234 

I use two Deco M9+ units at either end of my house connected with powerline (Ethernet over Power EoP) adapters in between and I use them in router operating mode. This is because my house has brick internal walls which cut the wifi down a lot. I believe the M9+ units will use a wired connection over wireless wherever possible on the expectation it will be faster. Recently one of my EoP adapters failed (protip: don't put them upside down) and when there was no ethernet backhaul the M9+ would fall back to wifi backhaul.

My connections are as follows: {Internet}-->broadband-modem-->M9+(main)-->network-switch-->EoP-adapter-->powerline-to-other-end-of-house-->EoP-adapter-->M9+(extender)

As you can see I don't have another router so am using the M9+ main unit in router mode.

I hope this helps a little for you to retry router mode.