לזכות משה יהודה בן פעשא דינה
ור' יהודה יעקב בן דינה חאשע
As we remember from the previous post - Rav Elchonon wanted to explain the opinion of the Maggid Mishna that we may do anything for a dangerously ill person even what is not necessary to save his life as follows: Just as we permit a Mohel to cut those parts of the foreskin that are not critical for the mitzva of Milah as long as he is involved in the mitzva, so too we may do anything the ill person needs as long as we are involved in the mitzva of saving his life.
Rav Geshtetner poses an additional objection to the one we mentioned in the previous post [that cutting off the ציצין שאינן מעכבין is part and parcel of the mitzva of מילה and therefore may be perfomed on Shabbos. But חילול שבת for a non-critical need is not subsumed under the היתר of וחי בהם so we have no permission to do it!]: Bris Milah is a mitzva. When you do a mitzva - you should do it right, in the ideal לכתחילה manner which includes the ציצין שאינן מעכבין. So we permit that even on Shabbos. But the היתר of חילול שבת in order to save a life and fulfill וחי בהם, is not categorized as a mitzva but as a מתיר. Of course, it is a mitzva to save a life but the חילול שבת is only permitted because of the pasuk וחי בהם which is not a מצוה but a מתיר. So there can be no proof from Bris Milah on Shabbos to Pikuach Nefesh.
Let us see how he puts it: