I chose ambivalent because I interpreted "reverence for tradition" as a positive thing, compared to "frustrated...by the vested interests" as a negative thing. Also, I was trying to find an answer that was limited to the authors attitude toward lawyers, to the exclusion of 18th century law in general.

Why (E) is wrong: It's too big a jump to go from rightly admired them to infer an understanding of their motives. Maybe they admire them but their main motive was to resell them for a profit.

Would it be correct to translate the question to "Which of the following, if true, would make the conclusion invalid?"

Yes!