To me an accent wall is to serve a purpose and that purpose determines the location, For example, Mother's condo had a DR and LR open to each other, so we painted the walls a beige tone. But the LR wall opposite the DR we painted terra cotta and the DR wall opposite the LR we painted blue (colors came out of the drapes which were the same fabric in both rooms). That way, each room looked to be separate and had a different feel, but coordinated. In my old LR, I had a large window with dark green drapes on it while the walls were a soft beige. To balance the weight of the windows on that wall, I wall papered the opposite wall in a dark green with a gold stripe and had the piano on that wall. It helped balance the color in the room and it gave weight to the wall with the piano. Sometimes a room may feel to long and narrow, in which case you'd want to paint the end wall a dark color to make the room feel more square. So lots of factors, not just where the windows are....

You can go dark or light with the color - it depends on the lighting in the room and what you are trying to accomplish. What I would do is avoid more yellow since you already have it in the hall and in the light maple floors and furniture, and you need some contrast. Color is very personal and subjective. Look at example photos of bedrooms that are similar to yours and see what appeals to YOU. Personally, I would embrace the Scandinavian feel and keep the walls very light. You can add warmth and color through accessories and bedding. You can use lighter blue/greens Keep it neutral with soft whites Use deeper tans and blacks, but you need plenty of textures to make that work You can try something darker, but you need lots of light and high ceilings, imo.

I only see one sofa/bed and it looks black to me. How are they arranged? In an "L" or what? Is the striped thing a foot stool? Romans will work best with the rads under the windows. If you play the piano and expect to keep it in health, it should be on an interior wall. More pix of all of the space and furniture will help us help you. These charts on coffee and lamp/end table height requirements may help you finish up the project. Its a neutral room so add whatever color you love to it. Rug size will depend on how you arrange the sofas, so more pix will help.

I would paint it the ceiling color. But why don't you buy a it of poster board. Cut it to fit a small portion of it to fit into the squiggly space. paint one side the ceiling and the other the wall. Then you can test without painting wall. But personally, I would highlight it because its such a special feature and it will actually draw the eye up and make the ceilings look taller, in my opinion. Good luck!