I would put the fronts as you say either side of the TV about 30cm or so outside from the edge of the screen. This should put them around level with the outside of the main sofa (assuming pic to scale).
As for the rears you could move them forward of the fireplace so they are either side of the sofa but just a little back but not sure if the window will allow the correct position on the right. There is no real issue having the speakers up higher and pointing down if needed. The rear wall looks a little too far away for me although it may work (could you put the speakers back there as a temporary fix (place them on something) to see how they sound.