ujm: Thank you for prodding me to look up the sources again and not rely on my memory. I must take back what I wrote previously regarding Reb Moshe’s teshuva to the Yeshiva. In fact he never uses the phrase “b’shas hadchak” in the entire siman. What he does write is that one who is a “baal nefesh” should try to be machmir, a Yeshiva should be machmir, and one who lives out of town where it is difficult to come by CY SHOULD NOT BE MACHMIR. He does not even say “NEED NOT BE MACHMIR” he writes “SHOULD NOT”.
I’m afraid I do not have access to Rav Forst’s sefer, but I understand that it is identical to the teshuva in IM8 where he writes that one who lives in a city with a minhag to be machmir should not change his minhag without a shaas hadchak if it risks undermining the financial stability of the local CY producer. Since all 4 teshuvos printed in IM clearly consider it a chumra, I see no reason to suspect that Rav Forst found a letter to the contrary. If I see it I am happy to reconsider.