Firstly, I must let you know that I'm not Windy's personnel.
So, I can't answer all of your questions.
I don't know Windy's plans for the future.
About sky condition, NOAA uses the following definitions :
Turkish State Meteo. Service uses the following symbols:
(cloud coverage in octas as I interpret them)
My question to you:
What is the usefulness if Windy adds a "mostly sunny" icon?
"Partly cloudy" is not enough?
Do you think that the average reader
can see any difference between the following icons?