Very busy regarding to personal and business problems. I try to give some answers:
What actual parameters does this system measure -- just solar radiation or UV as well?
Only solar radiation
Will it enable me to discard my current UV sensor (UVN800). It seems to be a heap of garbage anyway
No, therefore it measures only solar radiation you need your UVN.
Is it like a thermometer in a jar and will it measure (or indicate) cloud cover?
No it won’t directly. Have a look at pic7
When set up, do I place it in a Stephenson's Screen alongside my existing temperature/humidity sensor or somewhere else?
I don’t understand what you mean. Sorry, not a native speaker.
Regards
Jose