Hi, Bli,
Good question. This is a tough problem because it's hard to anticipate what the author's would definitely disagree about in advance. In order to aid yourself with a good prephrase, you do need to note in what respects the passages diverge. You correctly note that Passage B discusses a social structure angle that Passage A does not. This insight could help you return to the passages to seek evidence something about which both authors would likely differ. This direct contrast can be found around lines 30-33 of Passage A and lines 55-59 of Passage B.
Thus, your reasoning is correct in part, but in fact you can find explicit evidence to support answer choice (A). As a side note, there will always be adequate evidence in the passages to support (adequately) one answer choice over the others.
With respect to answer (B), the author of Passage B would certainly agree with this statement, but there is no certainty about the opinion of the author of Passage A about this statement. For instance, the religious convictions outlined in Passage A could be construed as a set of "social convictions," just a different set than those described by the author of Passage B. By leaving "social convictions" just broad enough in answer (B) the LSAT has left uncertainty about this statement for the author of Passage A. This uncertainty is an intentional attempt to create a trap answer.
For a quick elimination tool, on your first pass through these answers, ask yourself what you know for sure about the response of each author. This is more or less analogous to a Point at Issue LR question.
(A)————Y—|—N
(B)————?—|—Y
(C)————N—|—N
(D)————Y—|—Y
(E)————?—|—Y
