I'm sure you are all gripped by this evolving story so here's another update
Got a Chromecast version 2 today.
Bit fidlier to set-up as it needs to connect to a router with internet connection (the internet can be disconnected from the router after set-up it seems). Not possible to set-up the device on a Mac , as you need to run Google Home app in order to establish the connections. Once set-up you can use Chrome browser to 'cast' tabs or desktop from Mac.
Much much better signal and framerate, you can change the output between 60 and 50Hz but not the resolution it outputs.
One major drawback is that it outputs HDCP, to be expected but a bit of a PITA.
Stripped HDCP by routing the HDMI through a cheap splitter box.
Next stage is multiple phones and dongles into the computer, I'll be sure to let you all know a trick I have up my sleeve in order to accomplish that