Please describe "dark". There are several things that affect how a room feels and functions. Lighting designers take into account finish material colors, ceiling heights, fixture spacing, foot candles desired on each surface for task, general lighting for the entire space, etc. One can have well lit horizontal surfaces with dark walls and the space will feel dark. This can be a common problem when relying on mostly down lighting in a space. The term "spot" is not very helpful because it is used too loosely. Some call all halogen reflector lamps "spots". A true spot is usually something much narrower than 25 degrees and it is unlikely that your electrician put in that narrow of a beam spread. The Juno IC928 housing with 604 trim will accept up to a 90 watt Par 38. If you are limited by circuit capacity try changing the lamps to Philips IRC 70 watt par 38 which give off similar lumens. But first you need to figure out the beam spread you have and need. The Juno IC20 housing with 205 trim cannot accept a 75 watt lamp. It maxes out with a 50 watt par 30. You'd need a 216 trim for that housing to take a 75 watt lamp. A lighting designer starts with what light is needed and chooses fixture types, lamps and spacing. Then they choose the fixtures to meet these needs. The fixtures must hold the selected lamps and trims. Non lighting people tend to choose fixtures first. Interior designers and lighting designers work together to create or furnish a space and the lighting for it. The reason I'm going over this is because you need to think this way but may have to make the best out of what you have to work with. In many instances you can end up needing to remodel your new house to fix lighting errors. A good lighting designer may come up with a few solutions at different levels of costs to choose from.

At that height, I don't think it is a chair rail, it is a picture rail. Those are unique antique fixtures and hard to come by. I wouldn't remove, but I would not have a separate color for the lower half. I would consider the wall all one and paint it all the same, and then use the picture rail to hang the boards. Does not really matter how you paint, either light or dark. Lighting and window will make most difference in how "light" the room is. Vibrant colors can also make a room feel more cheery, if not "lighter." Up to you which way to go. A little green can go a long way, you might consider some plants. I hate watering them and have a tendency to kill indoor plants, even though I am a horticulturalist. I am like the plumber who always has a leaky faucet at home. I tend to ignore my houseplants. So your idea of an accent wall with some greenery might be a really nice and easy way to make the office a fun space.

As you decide where to place your desk, first know how the sunlight enters the room at all hours of the day. A glare is more than annoying. Sit in the chair you will use before buying it. Make sure your chair is comfortable, has a high back agaisnt which you could rest your head and with back support where you need it and with wheels ... because they're practical. Get an executive's chair ... because you've EARNED it. Suggest you do not place your desk against a wall in a way that makes a wall your "view" from where you'll sit most of the day. Consider putting one side of your desk against a wall in a way that keeps the glare from the window(s) from being a problem but enables you to look up and away from your computer (and out the window) frequently enough to keep your eyes from getting overly tired. If you are right handed, you might put the left side of your desk/table against the approximate center of the left wall so you can face the window. You could put a credenza on the wall behind you so the credenza would be usable with your right hand, doubling the size of your desktop. A new overhead light and/or wall sconces could save you desktop space. Do swap out the ceiling fan for one with a light so you're not dependent upon only lamps for light during the shortest, darkest days.

Warning: (learning from just painting Grey in my living room that has loads of windows and light) Most Grey colors turn out much darker looking once you paint them on all the walls. We spent $2,000 to change the grey in our livingroom to a lighter grey because the first one turned out too dark and that was a grey that was much lighter than any you have choosen. I had even gotten a sample and painted one wall to test the color but when the walls all reflect off each other it gets way darker! I would test Beacon Grey or Breath of Fresh Air. We ended up using SW 7071 Grey Screen You can see in this photo how much darker it can get depending on the shadows and lighting. Here it is in the day light: During the day it is almost baby blue but at night it is VERY grey.