I am not sure for INS, but I believe that env_sun and env_moon are the sun and moon entities (in l4d anyways)
If not, most likely an env_sprite.
If you know how to use hammer, open up the map in that and check out the entities, or do a stripper_dump in console and search the dump file for sun or moon, and edit accordingly. As for lighting, environment_light is indeed the light that comes from the skybox. To change skybox texture, edit the classname worldspawn, again look in the dump file, I would write out a cfg but my phone has no special characters, but il check in tommorow invade you can't get it.