Thats a good point I missed about stitching at 2000x1000 to 2800x1400.
Looking much better. Now experiment a little with the aperture one stop up ( bigger,,,F6). This will still give good sharpness but let twice the amount of light in at the 1/2 shutter speed. But only if you are in the low light situation. If you turned on all the overhead lights you probably wont have to do that. It also looks like the shades were closed a bit. Open them so the shades focus the light up and not down. This will bounce light off the ceiling giving you more overall lighting in the room. Shades down just make hot spots on the floor and furniture.
I noticed in the example none of the overhead lights were turned on. Turn on ALL the lights if you can. This may make the image have a yellow color cast. At the bottom of the layers pallet click the "adjustment layer" icon and choose "photo filters". Pick the "cooling filter 80". Now click the color box that is blue and select a much lighter shade of blue. Move the slider to the right to increase the effect until the image looses the yellow cast. Dont go too much or it will turn green/blue.
Or you can choose "color balance" instead of the photo filter. This is a little more powerful. You can adjust shadows, midtones and highlights this way. These are my setting for color correction when the image is yellow. ( -5, 0, 5) set this the same for shadow, midtone and highlight. I Recoreded an action to do this so I dont have manually set it each time. If you make the action then run it once. If the image is still yellow run it again so you have two adjustment layers instead of one. Do this up to 4 times until the image becomes "cooler" or looses the yellow cast. Now click off the layers one at a time so you can see how much color correction you are doing. Too much will make noise on solid colors. Just click them on and off until you get the best color correction.
To lightent the image. With your test image shown open it in photoshop. Make an "adjustment layer" by clicking the littel icon that is a circle with half dark and half light at the bottom of the layers pallett. Select "levels". Move the middle slider to the left so it reads about 1.25. This will lighten the image and improve things a bit. Click the little triangle button in top right corner of the pallet and "flatten" the image. Then apply the "unsharpen" filter with about 35%. Or use the plugin smooth mentioned and you should see a dramatic improvement.