I am trying to describe a transitional fading of indirect light from the outside world (particularly from the skylight) at the cave entrance from full indirect lighting, to none. In my particular project, all underground is affected by skylight indirect lighting, therefore not dark at all.
See above.
Not necessarily, as having a single isolated source of light shine down a long well would create a spotlight effect rather than illuminate the cavern by too much.
I don’t know why I forgot to mention skylight’s indirect lighting in my original post, I apologize for the confusion.