I saw that you quoted from sefere Mechalkel Chaim. Where did you find a copy? I have been searching for one.
Did Rav Moshe write the author a letter in which he was chozer about his opinion to say gashem?
Binyamin Marwick, Baltimore, MD USA
INSIGHTS INTO THE DAILY DAF
(a) RAV MOSHE FEINSTEIN zt'l (IGROS MOSHE OC 4:40) writes that the Kamatz of "Tal" is correct because this phrase is indeed at the end of the sentence (as many Sidurim place a period after "Morid ha'Tal"). Accordingly, it is proper to say "Morid ha'Gashem" with a Kamatz as well, in contrast to the spelling in many Sidurim. Such an opinion is cited by the LIKUTEI MAHARICH. This is the way the word is punctuated ("ha'Gashem") in the authoritative Redelheim Sidur.
(b) Others, however, point out that the text of "ha'Geshem" (with a Segol) appears in all early Sidurim, of all Jewish communities, as well as most current Sidurim (except for those based on the Redelheim Sidur). Moreover, as mentioned above, the logical flow of the sentence clearly indicates that "Morid ha'Geshem" is not the end of the sentence (regardless of whether or not the printer placed a period there). According to SEFER MECHALKEL CHAYIM, Rav Moshe Feinstein himself retracted his opinion. Why, then, do most Sidurim spell "ha'Tal" with a Kamatz and not a Patach?
The author of SEFER SHA'AR HA'KOLEL, printed in the back of the SHULCHAN ARUCH HA'RAV (who explains the Nusach of the Sidur of the Ba'al ha'Tanya) suggests that the word "Tal" is spelled with a Kamatz because it is not part of the regular text of the Shemoneh Esreh as established by the Anshei Keneses ha'Gedolah (as indicated by the fact that many Ashkenazic communities do not say it, and the Gemara says that it is not obligatory). Rather, it was added to the text of the Shemoneh Esreh by the Mekubalim. As such, it is a separate insertion that stands by itself and does not continue into the next sentence, and therefore it has a Kamatz and not a Patach.<<
The Nusach Geshem was in all old Sidurim. It was changed to a Kametz by Sidur Vayetar Yitzchak and those who followed him such as Rodelheim. I have heard that in Lita the Nusach was Kametz, hence Rav Moshe's Psak. I have heard that Rav Gustman also said Kametz. The assumption is that this was all based on Vayeter Yitzchak. Vayetar Yitzchak was printed in Berlin in 1784 by Yitzchak Satanov who is generally regarded as a "Kal Da'as" and his opinions are believed to have been swayed by the Maskilim. He introduced many changes into the Nusach ha'Tefilah, but his Sidur was printed without any Haskomos. Today his changes are viewed as discredited, and most Sidurim are reverting to the old Nusach.
If I am not mistaken, Mechalkel Chayim is a small pamphlet which is probably not available unless one happens to know someone who has a copy.