Question 1: I've had my D800E sense May 25th and have just over 5200 pics on the hard drive according to LR 4.3. With that many pics, it takes up just under 200Gb's of storage. So get a 2Tb drive and you'll be good for storage for a long while.
Question 2: I've tested mine and think I have it to a small degree but have not noticed it in day to day use, even for paid small family photo shoots. So I wouldn't worry too much about this. I do think that the issue is blown up a little.... but that's just my thoughts on the matter.
Question 3: The D800 will do 5 fps in DX mode and 6 fps with a grip in DX mode as well. I've never felt I needed more then 4 fps to be honest, but you have a D700 that with a grip installed will do 8 fps if needed. Also to note, the D600 is a 5.5 fps camera.
Question 4: The D800 in DX mode will and does give a bit better image quality at the pixel level compared to the D7000. So could still use DX mode if needed, but most will shoot in FX mode and crop in post unless you need the FPS from above....
Question 5: The D600 is said to have about a one stop advantage at the pixel level compared to the D800. But unless you plan on printing very large, you may not ever see the differences.
Question 6: From what I've seen, lensrentals.com post some good insite on the matter. Here is the original blog:
So basically, it comes down to what you shoot and what kind of build do you feel you need. Also, keep in mind, the D600 is much more like a D7000 with a FX sensor in it and the body is a little bigger. Where as the D800 is bigger like the D700 and has a little better build then the D600.
Other things to think about is the shutter durability/speed:
Flash sync speeds are a little different but worth thinking about:
D600 = 1/200th vs. D800 = 1/250th
There are a lot of small things(specs) to think about when making the choice. I think if the D600 came out before the D800, I may have bought the D600 and then would have wanted the D800 because it better fits my style and personal needs. Overall, I'm very happy with my choice and am glade I spent the extra money.