When you open a NEF in CS, it launches Adobe Camera Raw to perform the raw conversion. In ACR, you can make changes to various settings. Once you accept the ACR settings -- changed by you or not -- and continue on, by default ACR creates a small "sidecar" file in XMP format stored in the same folder as the NEF. The next time you open the NEF in CS, ACR reads that XMP file and applies the settings.
The upshot is that you can reopen the NEF in CS as many times as you want, applying the original ACR settings (or changing them if you like) to get the image the way you originally adjusted it in ACR. Now, if you edit the image in Photoshop CS itself, you have to save it as a copy in another format in order to preserve your changes. Typically, you would save it in Photoshop's native format (.PSD) so you can later reopen it with all of the edits intact.
But you'll still have the "negative" -- the NEF file -- and the raw conversion settings -- the XMP file -- needed to reopen the original image without the Photoshop edits.
Note that in this workflow, you never make any change to the original NEF, only to the conversion settings in the XMP file.
Regarding ViewNX 2 and LZW compression: Yes, you can choose to save the 16-bit TIFF with or without LZW compression. LZW compression is totally lossless, so the only cost of using it is a bit of CPU time needed to perform the compression when saving and then decompression when opening the file. Given the speed of modern CPUs, this seems a small price to pay for the saving in disk space (20-30% or so, I think).