Because the sofas are big heavy and dark I agree that you need to lighten up this room. I wouldn't have additional furniture because I think it looks full already but I might swap your little table lamp for a standard lamp to you have a light that is the same height as the TV. I also think your mirror is undersized, it should hang portrait to make the room seem taller - a round mirror would also work. I would get a white dining table. The mahogany trunk seems to obscure the view of the fireplace- perhaps a lower table should be better. I would also get an oversized houseplant. I don't mind the fireplace. I think you can put any colour on the walls and I don't think it has to be neutral but you should replace the curtains with much paler-perhaps even white. I think one of the reasons for replacing the curtains is the now slightly dated pattern. Perhaps put pattern in art or cushions so it can be changed more regularly. I like the rug but it does dictate your decorating choices- could it go in another room? If it can I think that your instinct is right that a large pale textured rug would be comfy and visually break up the dark tones of the sofas and floor. Lastly I would add a throw and a couple of cushions to make the sofas less imposing, add pattern and break up their darkness.

The mock up looks good! One word of advice to make the mock ups look as good in real life- keep the place tidy, when you first finish the decor of the rooms make sure you find a place for every toy, boom, remote control and make sure everything is kept in its place. It will make the rooms look more streamline and contemporary

Because the tall units will be about 2.4m from the door I don’t think you have to worry about them being in your sight line…… but if you are worried you could of course put them on the opposite side of the room. However I always think that the usual 55cm chair seat height makes your eye line about kitchen cabinet height so you could feel a bit overshadowed by the kitchen island. In my opinion the solution is to put the kitchen at the dining end and then move the dining table towards the window. You might also consider dual purpose furniture- for instance instead of having a dining table and chairs plus a sitting area consider an upholstered dining bench that can be also be a daytime spot for coffee and reading the paper