Try posting a picture of what you are dealing with. On many tub surrounds, they are designed to be installed on the studwall, and when you then install the drywall up to it, you lap it over the flange. Without seeing what you have, and how it is installed, guessing from your description would be risky.
KerdiBoard is a tile substrate. It's basically foam with a special membrane on both sides that is designed to accept thinset. It would not meet building codes unless it was covered in tile, so no, you would not want to use it in the rest of the room where you wanted just paint. Underneath tile, it is fine, stable, strong. Just painted, it would not meet fire codes, since the foam would not be protected, and could burn plus, without the tile on top, you could dent it or poke a hole in it.