I guess I consider myself lucky. My shutter count is 2889 and I have only experienced a few specs of dust on my sensor. They were not in the usual corner location, but more towards the center. I change lens frequently, so that could account for it. But whatever I get on my sensor I am able to blow off with my Rocketblaster. I bought a sensor swab kit when I bought the camera, but I have not needed to use it yet (Crosses fingers). I really hope they solve your problem this time. Although I have not had to send my D600 in for repair, I know how it is to send something in and have to wait for it. Even worse so when there is an uncertainty of whether it will be properly repaired when returned.
Good luck, Bob.
Good luck, Bob.