You do not mention whether you wooden wall will be painted or stained wood. Either way, there paint products you can use to properly seal and protect the wood either way. You should visit your local paint store and tell them what you are trying to do and see what products they recommend.

I'd go for make it white; then right there, flop one nice little white overstuffed loveseat or something that fit, or one big whopping chair, and fit in a tiny table and light; treat that corner as it's very own micro-retreat, and the seating is to not face out into the room. Like little lounge spots put under stairs, make this one dedicated to the view; and make it minimal. Decorate the rest of the room as if the column and retreat bit isn't there.

I'd suggest dark red curtains if you need curtains at all -- the windows are beautiful as they are, and I'm not sure whether some kind of blind might not give you the ability to darken the room when you want to. On the wall, if you are real fans of any movies or TV shows, three big posters would be fun. Otherwise, I would still suggest using three pieces of art there of that size, all equal in size and somehow relating to each other. Three floral works, or three works in the same style, perhaps. I think a large plain textured rug in a light neutral from one of the cushion fabrics would tie the space together well. You might also consider a lovely low coffee table on the rug to lay out your snacks and drinks for serious media nights.

I have some suggestions if you are thinking budget friendly changes. Make a photo gallery with different size frames and arrange them parallel to the stairs, so you can enjoy each photo as you climb up. And for the area that will be on a level with your upper floor, try some pretty decal that will connect the photo gallery to upper level. And last, if you have budget add some beautiful hanging lights in the stairwell. Also think about repositioning your refrigerator.