I think that the sage panels will look great. I had the black checked ones in my living room years ago. I hung them with the tailored valance and instantly wished that I had hung the panels only. I like a traditional look, but the valances made my home look more old-fashioned/country cottage than I wanted. I think you've made a great decision to try only panels first. If already mentioned, forgive me, but I recommend that you use a long curtain rod and extend it beyond the windows. Hang the panels against the wall to leave as much window space open as possible. I'm all for natural sunlight! If you need privacy in the evenings, then sheer panels underneath the panels could be used.

You can pretty much use any curtain panel with ring clips. They do add length to a panel, so make sure you measure accordingly.

Notice the above valance -- the bottom of the valance is just covering the upper molding of the window. Inverted pleat too. Notice out it takes advantage of the upper space? Actually makes the window appear larger.

Between the radiators and no wall space to the left of window D, I would skip curtains and rods altogether. If the radiators are electric, you definitely want to skip curtains. You look to have plenty of depth in the windows to hang blinds or shades in them.