By way of an update......
I have now sorted the green FN key issue with a firmware update (yey).
But the rest remains unresolved. After several transactions of correspondence with the Akko customer services people they have essentially confirmed two things:
1. Their JSON file for this keyboard is "limited" by the rules of the VIA system. The response indicates this is a limitation around terms of use rather than what can be achieved with a more complex JSON file, but I could be wrong.
2. They have in a round about way confirmed that they have no windows driver support for ISO versions of the Monsgeek M1 V5 keyboard.
So it is limited VIA functionality or bust, sadly.
Perhaps inexperience on my part in the custom keyboard world, and perhaps not sufficiently drilling down into specifics. But in any case it leaves a sour taste that the keyboard does not work as advertised.
It is still a great keyboard, and I love the Cherry profile. It is great for typing! I'm just disappointed it's not got all the functionality I was expecting.
But you live and learn!