I would agree with you that the lack of a specific reference to international law in Passage B makes this confusing. I don't love the question, but let's see if we can work through it.
My best guess as to why the question you raised doesn't make E incorrect, is that the nomadic nature of the Roma people, traveling between states, automatically makes this an issue of international law. This might be a bit of a stretch, which is why your approach of eliminating the other answer choices was a good one.
You failed to eliminate D, which is an appealing answer at first. Neither author mentions a case-by-case basis, but I see how the tone of Passage A gives the impression that the author would prefer that international law is left out.
The best evidence against answer choice D is on line 39: "Because this element essentially grants the state the arbitrary right to decide if the Roma constitute a minority without reference to empirical characteristics, it seems patently unfair that it should be included in the definition."
The author does not want states to have the right to designate the minority status of the Roma. Because of this, Author B is against the "case-by-case basis within individual states," but approves of standard, empirical criteria governed by international law. One author might approve of D, but not both.
