I have a DR that is almost 13' square so mine is close to your size. My DR table (without leaves) is almost exactly 56" square. It looks great when no one is eating at it, but even without putting in a leaf (it will seat 5 a bit crowded, 4 better), it is hard to get around. I have a small sideboard on one wall, a chest on another wall, and no furniture on the 3rd/4th walls and it's still crowded. Serving is a nightmare as I cannot get behind the chairs to go around the table. If I put in a leaf (my leaves are about 24" wide), I can seat 6 comfortably. One Christmas, I had to put in 2 leaves, and the people at the end, well, one had his chair practically in the entrance into the LR, and the other was up against the french doors that lead to outside - it was awful and this was to seat 9 people! I cannot imagine how you can possibly use your table in this room, or any other table and hope to seat 10 people. It's just not big enough. It takes a BIG DR to hold a table that can seat 10 comfortably. My DD has a DR that is 15' x 16 1/2'. Her table is 52" wide and if she uses small chairs, she can seat two at each end and with two leaves in her table (a triple pedestal table), she can seat 4 down each side, thus 12 in total. One time, she seated 14 - it was very cramped - no b&b plates, single wine glass in the table setting.

It is large, but you can work with it. Get a rug large enough so it can run several inches under the sectional. Position the coffee table as you did in photo #2. A rug will “ground “ the coffee table. Drapes which hang on either side of window will make the wall look wider. Then address the t v and its console. Install the t v on the wall and get all the wires behind a floating panel. https://www.allmodern.com/--Barry-Floating-Entertainment-Center-for-TVs-up-to-60-inches-GOLV5217-L6-K~A000932575.html?refid=GX433453041335-A000932575_74734208&device=c&ptid=901238351209&targetid=pla-901238351209&network=g&ireid=94406068&mktdc=GXPLA&mktcm=442727836&mktst=106091579932&mktcr=433453041335&mkts=%7Bsk%3A%7D&PiID%5B%5D=74734208&gclid=CjwKCAjw_sn8BRBrEiwAnUGJDuuyldA9-0-7O9ACKj4SjEMBuM6cG9QEi0BK798Y2-YdfYQ5BSK9hhoC5XoQAvD_BwE

That is gorgeous!!! You have incredible style. And so does grandma! Two thoughts - Either, go with the “U” shape and put the grandma couch in a different space ( this is what I’d personally do because I feel like the navy one can be such a showstopping piece in a smaller space with not much else around it. ) Or you can skip the added piece and leave it as is. You do need a very big , heavy looking coffee table. I would do one. Again, stunning space!! Can’t wait to see what you do...

It's not too big...looks great. What's throwing it off the balance is the t v and stand that look so squat. Put the t v on the wall and style a taller unit so it corresponds to the heft of the sofa and the height of the coffee table. This piece is $238.00 from Wayfair. You can shop around and see if you can do better. If your handy, from the looks of the inspiration photo, you could make one with three stained boards. A suggestion, add an additional pair of drapes, some sheers, and extend the rod as far as you can without impacting the door. It will make the window look larger.