You're definitely right. I've already decided that the mirror has to go. It's 8 years old. I spent most of the afternoon trying to find a new one. No success. I could repaint the frame (wouldn't deal with the style of it, however), or just put it in another room. I need something with a simpler frame in either gold or wood tone. If the mirrors at Stenella weren't so outrageously expensive, I would've bought one there. They had a couple at Homegoods that were fake bamboo, but they were too small or too over the top. I like the height of the existing mirror for reflecting light and breaking up some of the dark blue walls.

It depends if it's an interior door, or the front door. If an interior door, if you wall it off, it looks like you might obstruct part of the door frame. I don't know if the other posters are talking about walling it off in a "box," or creating an entire fake wall across that wall, which which case you would need to either move the baseboards, or get new baseboards - and those are old baseboards and you may have to have them custom made - even though they are not fancy baseboards. As an alternative you could repair the holes in the floor and ceiling and paint the pipes the same color as the walls. A good drywaller should be able to repair the open ceiling corner. If you wall it off in a box shape, it could have the potential to look cheesy. In my previous house, it was an old house like yours and had the furnace pipe running up from the basement through a corner of the bathroom through the roof. I painted it the same color as the wall and it looked fine. May be difficult to get in the corners behind the pipes, but it's doable.

Once you have furniture in your space the chair rail will not be noticed on that side. It will not be the focal point. Your furnishings and artwork will be what people will see. Also, spaces that are completely balanced are not nearly as interesting as having a little asymmetry here and there imo.

What are the furnishings in the rest of the space ?