Repetition of details keeps a home cohesive in design. Doing details here and there is like just going through your jewelry box and picking out the shiniest thing possible whether or not it works with the dress you're wearing. I don't think this dress works with too much frou frou. Skip the expensive coffered ceiling detail that most builders don't get right anyway. The wainscoting detail could add a nice touch to the home, if it's repeated in several locations to tie it together. And if it's done in a traditional manner, where it's not too tall (as is all too common), it can bring in some of the traditional detailing that you want to see in the home.

My MB ceiling is the same as yours and we've always called it a tray. There are double trays too. Our MB walls are painted BM Pittsfield Buff then we've got crown moulding. The flat part of the ceiling surrounding the tray is white and the angled part of the tray is also Pittsfield Buff. The tray itself is the next darker shade on the same color strip, BM Yorkshire Tan, if I'm remembering correctly. A decorator selected this combo. She said that painting the flat part of the tray a darker shade makes the ceiling feel higher.

Same color would be pretty - and easy. When you cut or adjust a formula, you're creating a completely different color. Every dimension of the color is changed, hue, value, chroma, LRV. The new color might resemble the old color in some way, but there's no way to know until you have it mixed, dry it out and look at it. In the case of Agreeable Gray specifically, if you cut it by 50% you get a light gray that looks a lot like SW 6000 Snowfall. Agreeable Gray and Snowfall don't look good together - they don't "go" together. If you want a color that looks like Agreeable Gray, just lighter try White Heron SW 7627 for the ceiling. If you pull all three chips, Agreeably Gray, Snowfall and White Heron, you'll see what I'm talking about.

I struggled with same question when we built. My walls are BM Pashmina and I painted inside my cells a slightly lighter color although when you look at the picture it looks the same (Dependent on the lighting). Houzzers recommended to me using the same as wall color or use it at 50%. I would just do as IdaClaire suggests and use wall color. With the height, it can handle the darkness of the color. They also recommended the vertical piece to be considered wall and to be wall color. All white is very pretty as well- just depending on what you like. Good luck!