Difficult verbal question need clarifications

GRE Verbal Problem Solving
1

The demands of ____ notwithstanding, a page or two in Dahl’s recent book on democracy that considered what public-choice economics has to say about “democratic failure”—or at least a clear signpost to that literature—would have been very well spent.

  1. clarity
  2. brevity
  3. comprehensiveness
  4. economy
  5. cogency
  6. thoroughness

it shows that the correct answer is A and B, but I still tend to choose C and E. Could someone please explain to me the reasoning behind this. Thank you.

2

The sentence suggests that the author of the book was so _____ that they didn’t give enough attention to some needed information (a page or two in said book…would have been well spent)

This suggests they were focused on being brief