It's too bad the heat register is right there. Makes me nervous to put curtains there.
You could put a curtain rod across the top of the three windows - looks like someone already had something there. I see little hangers on the corners. I wouldn't try to cover the top radius window with that. Why do you think you couldn't hang a curtain rod across the top of the 3 windows ? It's done all the time. There are also special covers made too match that go over windows like your 1/2 circle one. Look through some catalogs or websites. JCPennnys, Sears, there are lots more I can't think of right now, but I've seen them. Google decorative window treatments & see what you come up with. Or look for a custom seamstress to make them, my sisters mother-in law made something for her odd shaped window.
Or cover the bottom 3 windows & do the same treatment on the 2 seperated windows on other wall & leave the 1/2 circle open. Unless the light will bother you, nobody's looking in that one.