Block the space between the studs where the screws will go with pieces of 2x10 or 2x12. If it's an inside wall, double them up to let you sleep soundly!

I was at a friend's house and they had a built in similar to yours. They had the TV on a sliding shelf in the built in and pulled it forward for viewing , but then had the option of pushing it back all the way and actually had doors that shut to hide it. That may be overkill to close doors to hide the TV, but it looks like you could have a retractable shelf in your built in for the TV. Can you do without the loveseat in the room? I think the loveseat interrupts your flow into the room, both physicially and visually. Instead, how about moving the chaise where the loveseat is and maybe getting a nice upholstered swivel chair to put over on the fireplace side which could be turned to face the TV or the fireplace. It could be placed close to the couch and angled in so it doesn't block the fireplace. Or, if you aren't set on having a chaise, an upholstered swivel chair could be placed on either side of the couch-nothing too big. That would leave the entryway less blocked. I think that would made the room look larger. I really like to look into a room and see an unblocked expanse of floor all the way to the other end, if possible. With a chair on either side of the couch with a small table between each chair and the couch, you'd only lose one seat from what you have now. If you really want something to recline in, consider a smallish lazyboy for one of the side chairs. We love our leather lazyboys. Also, I would move the grouping as far back toward the breakfast nook as possible. I love the built ins, the fireplace and the glass doors out to the deck. It's going to be lovely when it's all furnished.

Top of the artwork on both sides should form a straight horizontal line. In this case I would make that a small but noticeably lower than the top of the TV. Because you are framing the fireplace with the same chair I would continue the symmetry with your pictures. In this case 2 on each side. They don't have to be the same pictures but they should relate. I'd also suggest making all the picture frames a darker tone to provide contrast to your wall color. Just my take, I'm a fan of symmetry when you can make it. It's more soothing on the eye.

I love your roman shades and the panels would be completely unnecessary. It would just add another layer of texture/softness/interest to that wall. If you did do something like that, extending the rod further toward the side walls would balance the windows by taking up some of the extra wallspace on the sides of the window. However, leaving just the shades would be just fine. I just feel you don't need anything else on the walls on the sides of the windows. I could see 2 matching poufs or benches (maybe caramel leather) under each window as a nice filler that would add seating or foot rests.