Photoshop various questions

Hi, I have many Photoshop questions! Any advice would be appreciated

1. I usually view my photos in Nikon ViewNX2 and also Bridge. But I just looked at some folders in “Finder” (Mac). Some folders only have XMP files for the RAW files that I edited. But some folders contain XMP files for every single RAW file, even though I had not edited every single photo in ACR. Why is that?

2. Currently I’m using CS6 on a friend’s computer. But what will happen when I return to CS5 on my own computer and try to open the CS6-edited RAW files again in CS5 ACR? I know that CS6 can read CS5 okay, but how will CS5 read the CS6 ACR files?

3. After editing RAW files in ACR, I then open them in Photoshop and either save a JPEG, TIFF, or, if having done further editing, I save a Photoshop file. But when I come to close the RAW file that’s open, I get the option to “Save changes to the Adobe Photoshop document ‘DSCN0001.NRW’ before closing?” What does this mean, considering that the ACR edits are already saved in an XMP file and I already save the final version as a JPEG, TIFF or Photoshop file?

4. I have my camera set to Adobe RGB. I understand this would be better for printing in future. But I don’t print much these days. Mostly email. However, it’s good to keep one’s options open I suppose, as I know down the line I will be printing things. What is the best decision to make here – RGB can be changed in Photoshop anyway, right?

thanks