Good job answering the question correctly, (E) is definitely more in line with the main point of the first paragraph (and the passage as a whole).
Answer choice (D), on the other hand, describes the purpose of the art critic's refusal to believe the painting was a forgery as emphasizing that "the concept of forgery itself is internally incoherent". Did the passage discuss any internal incoherence contained within the idea of forgery itself? No! The author does not question the definition or idea of forgery, rather, the author discusses the difficulty associated with determining the artistic value of a forgery compared to the original and offers an explanation (Lessing's answer). Since there is no discussion of forgery being internally incoherent, (D) cannot be the reason why the author introduced the highly reputed critic's persistence.
Comparing this answer to (E), we can see that (E) does relate to the discussion of the passage at large. Multiple points in the text support the idea that skillful forgeries raise difficult questions for art critics (lines 1-5, lines 21-27, etc.). Equipped with the knowledge that the author believes skillful forgeries can pose difficulties for critics, it becomes much easier to see that the reputed critic's persistence is just one such example of critical difficulty with forgeries!
I hope this helps