Quite simply... Is there a way to display a specific weather in the render window? I'm attempting to tone down the brightness of night fog in the Reach, since it's nearly as bright as day.

Sure, if you open up the weather you wish to change, there is a checkbox for "display in Render Window," I believe towards the top right of the window. Make sure you have sky turned on in the Render Window and you should be good to go!