How do I disguise a built in wall to wall cornice board?

I don't see any reason why you can't mount the drapes there and I would add the crown molding right across it.

Is the cornice open on the bottom or a closed box? I have to say that I would take the cornice down. It stands out from the window wall and darkens that end of the room. I also think the drapes would look funny being away from the window and the crown molding would look strange . It would require a little more work to remove the cornice and to patch walls and maybe ceiling, require painting. I think it would look so much better and add height to the ceiling. Just my humble opinion.

If you can remove the cornice, do so. If not, mount sheers UNDER the cornice just over the window, and the drapes ON TOP of it, going from the ceiling, and from the wall to the edge of the window and sheers. This way, the distance from the window to the cornice is filled by the sheers so it doesn't look odd. And yes, you can put sheers over blinds.

You can do what you stated with the draped and crown molding but I agree with Kal. I would see if the cornice boards could be taken down. The drapes and crown molding would fit much better if the boards were gone. Just my opinion, but I also think it would liven up the room.

I would think that drapes with deep folds would also help.

Take it down. Nothing is going to make it look good.

If closed, I would hang drapes at window top on the wall and omit crown molding over the cornice. Are there electric wires or pipes being covered by this cornice?

I dont think so. This is on the top floor with a low sloped roof so shallow attic. We have a speaker on the cornice but it is below the closed in area. The cornice bottom is open for about 6 inches then closed above that.

Thank you for your ideas. I will probably look at having it removed if not too costly.