What a lovely, bright room with so much potential! IMO, if you can find another location for the "gaming" do it :-) Given your sightlines I'd absolutely focus furniture layout around the fireplace. Here's what I'd do: Float a 3 seater sofa to the L of the fireplace, centred in the room. Small side table for a reading lamp on the L (nothing on the R arm side). Behind it place a console table for plants to take advantage of the lovely large window (if you need to add lighting, place it on the right side of table with cord running to the left), coffee table in front of sofa. To the R of the fireplace, a small side table and an arm chair angled out, in the space between the 2 doorways, another arm chair facing the fireplace with a small side table & lamp (play with which side works best for flow into the room from front door). One seating area focused on fireplace. Moving to the wall with 2 windows: create another seating/conversation area with a chair in front of each window and a low table in between - angle the chairs slightly towards each other. The chair in between the doorways "links" the 2 conversation areas. Use a large area rug to define the fireplace conversation area (at a minimum, front legs of sofa, of chair in between doorways, and chair to R of fireplace should be on the rug, no rug for the seating area in front of the two windows). Good luck and be sure to post your "Afters".

Exactly how narrow is it? A sectional isn’t going to work well in a small room. (Unless you buy a super-small sectional.) At first glance, I’d do the placements opposite of what you have sketched. (The windows will glare on the TV.) Edited to add: I studied your pictures a bit more. Is this room a former single-car garage? So it’s maybe 12’ wide?

Turn your sofa to face the TV/fireplace. You may not have any space between the wall and arm of your sofa. Maybe get a narrow table for behind the sofa for lamps. If you get a new sofa, don't do crazy oversized -- 84" is plenty long (I bought a 3-seater that's less than 72"!). Place your loveseat where they had theirs, next to the sliding doors. You now have L-seating great for conversations, and TV. I would suggest a sofa or loveseat with 2 chairs or one larger chair & ottoman if you purchase new instead of a sofa AND loveseat. And don't buy a sectional! If you could get dimensions of the space and your furniture, and photos of your furniture, that would be helpful. Their set-up looked great, though I don't understand the sofa placement unless sizing was an issue.

Here's the 9"-10" deep cubbies and shelf. It can be just a shelf and higher if necessary. I like the cubbies because it organizes stuff better. When painting the alcove, you may want to trim it with a simple molding along the front edge--also could be your kid's favorite colors. The lighting sconces don't have to be hard wired. IKEA has a good selection for plug-ins. I was thinking you also have that difference on the right where the bed extends near the closet, where you could rig up a rod and hang Manchester's flag vertically. I love kid's rooms, my son, now grown, had a cool room but I couldn't get rid of all his friends!