Regarding your Benjamin Franklin post - my father's father, Rav Nissan Waxman, wrote an article entitled, "Sefer Shenitalem Min Haayin - Al "Cheshbon HaNefesh" Umechabro". The article was printed in Heichal Shlomo's Shana Beshana in 1969, pp. 303-315. In this article my grandfather argues with those who claim that "Cheshbon HaNefesh" is a translation of Benjamin Franklin's memoirs, and (along with a dose of criticism of Lefin's occasional inappropriate literary style) reaches the conclusion that Menahem Glenn reached, as you summed up, "Basing himself on Franklin's method, Lefin wrote an original work that elaborates passages of Franklin's autobiography and expands upon his ideas."

On a similar topic, I am attaching Rav Yoel Katan's article (PDF), printed in Hamaayan, Nissan 5752, which deals with a maamar of Rav Dessler given in Ponevitch during Elul 1949 and appears in Michtav MeEliyahu, Vol. IV, P.234. Rav Katan claims that it is based on Dale Carnegie's writings. Actually, before I read this article I heard a rumor about Rav Dessler being fond of Dale Carnegie's writings and I called Rabbi Aryeh Carmel in an attempt to clarify. Rabbi Carmel confirmed the rumor telling me that Rav Dessler felt that the writings of Dale Carnegie are beneficial to the avodah of Mussar. I asked Rabbi Carmel if Rav Dessler read the books themselves and he clarified, "No, Rav Dessler didn't actually read the books, but rather he read an article in Reader's Digest which gave a synopsis of Dale Carnegie's principals." (At one point in our conversation Rabbi Carmel's wife, who suspected that the person on the other side of the line was having a difficult time swallowing the information, yelled out from the background, "Tell him - Mekol melamdai hiskalti! Mekol Melamdai hiskalti!")