It looks like you are trying to fit too much furniture into the space. I would get rid of one of the pieces.

If you're absolutely set on making all 3 pieces fit in the room, it looks like your only option would be to float the shorter loveseat against the windows. Pull them away slightly, add a table in the corner between it and the couch on the long wall, and move the chair to where the loveseat is in the picture above. Thankfully, the furniture backs aren't high, so them obstructing just a small amount of the windows won't look so bad.

For curtains, go with something airy, since the fabric on your couches is heavy and dark. I would go with a light colored pattern rug, maybe a trellis, or quatrefoil, and get a nice piece of artwork for the large wall that brings in an accent color - purple, teal, emerald, etc.

A table lamp for the corner with a mirrored base would look nice, and a taller floor lamp near the fireplace where the pillar candles are would look good too - although I don't know if there is electrical on that wall.

No, if you remove anything, don't add any more seating. An 11'x11' room isn't gigantic, nor is it small, but the location of the doorway, windows, and fireplace makes it tricky to arrange. You just have too much furniture in the space. That chair would look great in a bedroom or in a little reading nook somewhere else in the house.

I agree that you should remove the single seater. It would let you move things around a bit to give a bit of breathing room to the remaining furniture. I think the room would look better with blinds or roman shades as opposed to curtains. Then some lighting and artwork will warm up the space.