So is it possible for standalone version to change only the light (and not weather) part of environment.xml, or is it technically impossible not to change the whole file?
I think it's possible, you can simply just not include the clouds/weather/season part of the xml and just have it in it's own seperate xml but I'm unsure how to make the map.xml load the now two seperate environment files. The caveat to all of this is that the clouds/weather are responsible for assigning the appropriate cubemaps to the environment. Without it, or the wrong cubemaps will make objects look very out of place.
So what I tried to achieve the split of weather and light: I edited your xml to contain just the lighting data and copied it, along with the other xmls and cubemaps, to folder of Additional Game Settings mod with custom lightings. Now it is easy to turn it on and it seems to work, but I need to try it with maps with nonstandard weather like Rennebu to see if it has the intended effect :)