Some of the signatories were:
R’ Tzvi Pesach Frank, R’ Yechezkel Sarna, R’ Zalman Sorotzkin, R’ Yechiel Michel Tikuchansky, R’ Shlomo Yosef Zevin, and R’ Shlomo Zalman Auerbach.
None of the people you mentioned ever signed a letter saying such a thing. You got that information – or whoever told this to you got it – from the “hatekufah hagedolah” of Rabbi Kasher, who falsified the entire episode as well as doctored the letter.
RZW comprehensively debunks RMK’s statements (p.231) that “k’mat kol gedolei hatorah vechol RY’s bo’oretz” accepted the concept of ADG, and (Ha”H page 387): “…kovu v’ishru 200 rabbonim miyisroel kimat kol rabbonei ho’oretz gam chavrei Agudas Yisroel (milvad HaNeturei Karta).. .hashkofas daas hatorah merabonei ho’oretz bli pipukim vechashoshos…shehakomas medina hi…kehashgocho protis min hashomayim K’ASCHALTA D’GEULA.” As already mentioned above, RZW says that these quotes from HaH are regularly used by those who need it, to prove that the Gedolei Yisroel accepted the ADG. (Indeed, in HaH that RM Kasher himself considered this KK so important, that he refers the reader to it – **right at the beginning of his book** – even before his Hakodomo.) RZW continues, that not only did he speak to the Gedolim, who denied ever signing such a KK, but – after much effort – found the original document – with the signatures…and of course the document with signatures NEVER has the words “Aschalta DeGeula” on it! The actual words there are (reproduced in his book): “…hanitzonim horishonim shel KIBBUTZ GOLIYOS…” (The HaH version: “…hanitzonim horishonim shel ASCHALTA DEGEULA.”!!!) (Incidentally, RZW adds, that at that time no one yet had any idea that this “kibbutz goliyos” would also cause with the mass Haavora al hadass in the Olim camps.) RZW notes (p.144) that his criticisms of the HaH were originally publishedin the Z’eirei Agudas Yisroel monthly Digleinu (Shvat 5738) – during the lifetime of RMK, who obviously wouldn’t or couldn’t respond. (This is despite the fact that at the end of his foreword, he invited comments.) Later on (p.282) in his book, RZW brings further evidence, that RMK’s biasand prejudices caused him to censor/misquote and misrepresent facts in anarticle in the rabanut publication Shono Beshonoh, in order to give the impression that his pro-zionist views were not in conflict with the majorityof the Gedolei Yisroel. In page 286 he also shows how RMK in HaH distorted the words of the Gerer Rebbe (Imrei Emes) z’l at that meeting.
Another person who published (in 5729) an attack on RMK is Rav Moshe
Sternbuch shlit’a who was then a Rosh Kollel, living in Bnei Brak. His main aim is the Kol Hator which RMK attached to HaH – claiming it is the work of Rav Hillel Shklaver z’l purporting to be the views of the Gr”o z’l on Inyonei Geula etc – which somehow fit in very nicely with the views of HaH. RMS notes that the clear evidence that the entire sefer is not from the Gr”o or his students is the fact that it contains many modern Hebrew words and it is therefore unclear what is from the original and what was added later. In his opinion KH should not have been published – being a “Dovor She’eino Mesukan”. He also expresses his surpise at RMK who ignored the Cherem Hakadmonim issued by the Bes Din of Vilna after the petira of the Gr”o not to publish anything in his name without the haskomo of the Bes Din.. . RMS continues that RMK well knows the opinion of “rov minyan ubinyan gedolei hador hakodem vedorenu” (including RC Brisker, REC Meisles, RE Wasserman, RBB Leibowitz,RA Kotler and most of the gedolei Hachasidus) on these matters. But he disregards them and only brings those who are leshitoso. RMS then goes on to prove that even in this version of KH there are many rayos which clearly disprove RMK ideas in HaH and goes as far as calling him a ‘megaleh ponim beTorah shelo kehalocho”! His ‘maamar’ runs approximately 10 pages with point after point disproving RMK’s pshat in the KH and the Gr”o.
Hayotze Lonu Mizeh, that it’s more than obvious that when it came to stand up for his prejudices, RM Kasher was quite prepared to openly and/or surreptitiously doctor, censor and distort the facts. Thus, LAD, his book should not be used as serious proof for any debate on matters relating to the medina and the views of the Gedolei Yisroel. And, as mentioned previously, all his rayos etc misforim vesofrim must be double and triple checked – before being quoted as “Toras Emes”. It seems that this need for distortion and misrepresentation shows that even this renowned Torah scholar felt that without it he could never convince the (Torah) world that an independent medina prior to bias hamoshiach was the ideal choice of the recognized gedolim.