Upon flipping around the web, I've come to the conclusion that the "incómodo" of the otoño is due to the constant changeability of the weather which makes it a permanent "characteristic", so,  ser. I guess if the the summer is always consistently hot then ser might be applicale also: en verano, el tiempo es insoportablemente caluroso.

I think you have a choice of whether to use “ser” or “estar” depending which meaning you want to convey. 

 To me it is similar to describing a piece of fruit. Remember that if we say “las papayas están dulces” we are referring to a particular group of papayas, and is we say “papayas son dulces” we are referring to papayas in general.  

It could be the same for weather. “En otoño es ventoso” to refer to a general trend and “está ventoso, este otoño” to refer to this particular autumn.  

You are correct about estar vs hacer. Kwiziq has  lesson on this.