1. Yes, box sets can be entered on GR, even if they don't have an ISBN (in which case that field would be blank.)
2. It should not be combined with any single book, but could be combined with another set that includes the exact same works. It could be "linked" to the individual works if a "series" is created that would include all the individual works and the box set. But I don't think a series would be appropriate for these works according to GR policy (they seem to be completely separate works that were packaged together; please correct me if I'm wrong.)
When I say that they are separate works, I mean that they were never part of any nonfiction "series"; they were all published individually at different times, with no connection to the others, then later collated into a thematic set...right?
2. It should not be combined with any single book, but could be combined with another set that includes the exact same works. It could be "linked" to the individual works if a "series" is created that would include all the individual works and the box set. But I don't think a series would be appropriate for these works according to GR policy (they seem to be completely separate works that were packaged together; please correct me if I'm wrong.)
I'll look over it and make corrections as needed.