I am not sure I understand the question.
You can add fill light to a raw image in the Raw refine screen which is saved when you exit that screen.

You can add fill light fromn the image edit screen which is also saved when you exit that screen.

You can easily adjust again by entering the refine screen from the edit screen using the little green button, see snap.

Fred

Correct. The image edits are saved as a separated file (.flt) in the same folder as the image  which is applied to image when it is viewed or printed.
Yes. Create the edit ("Filter") on any image that has no existing edits and save the Filter from the Image Editor File menu. Give the Filter a suitable name.
You can use that custom filter from the editor but the easiest way is to right click the thumb and select Predefined Filter ...
You can use a particular filter as a Global Filter, or as a Print filter which is applied when printing and does not affect the appearance of the on-screen image.
Terry