1)Do you know where Rabbi Samson Raphael Hirsch quotes Moshe Mendelson? 2)And does that mean he held of him somewhat?
ב"ה Shalom Your question has a few aspects. One just asks for information about where Rav Shimshon Raphael Hirsch zt"l relates to R. Moshe Mendelson. RMM. However, there are other aspects as well, which include our attitude to "Gedolei Yisrael" and then our attitude to those who strayed off the path. I will start by saying, that Rav Hirsh is and was held in the greatest esteem in the Jewish world.. It would suffice to quote just a bit from the "Haskama"- letter of approbation written by Rav Yitzchak Elchanan Spector zt'l to Rav Hirsh's book "Igrot Tzafon" who calls Rav Hirsh, "Harav Hagaon Hatzzadik…". In Rav Hirsh's book "Igrot Tzafon", letter 18, Rav Hirsh speaks of Mendelson with sharp criticism. He says of RMM that one the one hand he acts in regard to fulfillment of Mitzvot with extreme orthodoxy but at the same hand acts like an "Ashkenazi Plato". He continues to say, that one must recognize Judaism from in itself from the Tanach, the Talmud and Midrashim. From within, we create the Jewish view of about G-d, the world, humanity and the Jewish people. , Yechiel Yaakov Wienberg zt"l , in his "Sridei Esh, Vol. IV, pg. 367, deals with issue at length. So did Rav Hirsh "hold of him"? Rav Weinberg, called RMM the philosopher, while he says that Rav Hirsh respected him although he criticized him. .RMM is quoted in the Pitchei Teshuva 357:1 as רמ"ד which is the acronym for Rav Moshe Dessau, (since he was from the city of Dessau, Germany). He also referred to as רמ"ד by the Maharam Shik in Likutei Teshuvot of the Chatam Sofer, and he adds that he looked in his translation to German of the Torah, and found no "epikorsut". So, on the one hand, people knew of his vast torah knowledge for which they showed respect , but at the same time they had their reservations. I would assume that is why he is quoted only by an acronym to leave him somewhat in obscurity. In the same Teshuva of the Maharam Shik ,he says the Chatam Sofer related to RMM as an "epikores". (See Also שו"ת ציץ אליעזר חלק טו סימן לב, תשובות והנהגות כרך א סימן תקמא, שו"ת להורות נתן חלק ז הקדמה) So it seems some still left RMM within the community and others saw him as an "epikores". Until this very day, we have many Rabbis, who shall remain unnamed, who are recognized for their great Torah scholarship, but because of certain opinions they have, because of their Hashkafa, because of their affiliation, or because quoting them within certain communities will create political fallout, they are not quoted in their full names and/or their books are not learned. And I am referring to cases in which all would agree these Rabbis are not Epikorsim. Though unrelated directly to your question this situation reminds of what Rav Avraham Zacutt zt"l, (author of Sefer Yuchasin, who lived at the time of the expulsion from Spain, who was a Rabbi, an explorer, astronomer, and a historian of the Tana'im, Amoraim,and later Rabbis) had a dilemma about. Should he include Elisha ben Avuya among the Tana'im in his book or he is "out of the camp" so to say? Although, others differed, he chose to include him among the Tana'im since he didn't cause others to sin., and Rav Yehuda Hanasi included him in Pirkeui Avot (4:20). All the best