Why with regard to all the mitzvos, פיקוח נפש is דחויה, while on Shabbos it is הותרה? Also, how do we know that נמסר בידכם - Shabbos is given over to your hands only with regard to life and not with regard to Yisurim and other things?
First we will say the sentence and then expand: In the entire Torah there is a contradiction between קדושה and לכם. If it is הקדש then it is not לכם. Only on Shabbos is the קדושה considered לכם! That is the pasuk קדש היא לכם. That is why פיקוח נפש is הותרה on Shabbos.
Now the explanation: We know that there is no reward for mitzvos in this world. שכר מצוה בהאי עלמא ליכא. The guiding rule is היום לעשותם ומחר לקבל שכרם. Today we work and tomorrow [in the next world] we receive reward. However there is one mitzva where it is completely transparent that the reward is not in this world but in the next. That is the mitzva of קידוש השם and יהרג ואל יעבור. Here one dies for the mitzva so of course the reward cannot be in this world.
There is another mitzva where we hear echoes of reward in this world: That is the mitzva of Shabbos. Shabbos is מעין עולם הבא. In other words - we taste the eternal reward of Shabbos when we keep Shabbos here. So it can't be that mitzvos are merely דחויה by Shabbos. Shabbos says LIVE. BE REWARDED!!! It is YOURS! The world of reward, of eternal life.
So now we return to the question of why we mention עונג שבת in מוסף when עונג שבת is from the נביא while מוסף is מדאורייתא. Indeed, the specific types of עונג such as eating and drinking are only מדרבנן based on the נביא. But the CONCEPT of עונג is from the Torah. Shabbos is נמסרת היא בידכם. Shabbos is the world of reward. Shabbos is pleasure such that there is no room for dying. פיקוח נפש is הותרה. So of course the mention of עונג belongs in מוסף.
Now we can understand the uniqueness of קדושת שבת vis a vis all other types of קדושה. Other קדושות don't CANCEL or suspend other things. The Land of Israel is holy but it is not מבטל anything from other lands. The Bechor is holy but doesn't nullify anything from other animals. Shabbos, in contrast, nullifies and cancels the essence of weekday time. Shabbos is the next world which is a contradiction to this world. הבדלה is NOT as we thought [at the beginning of this essay], the other side of the coin of קידוש. Kiddush [we thought] says that this time is separate from the rest of the week and הבדלה says that this time which is now ending is separate from the rest of the week. No. קידוש sanctifies the day, nullifying weekday, mundane time. This should go on forever. Now time is holy. הבדלה is the STOPPAGE of this holy time.
קידוש is ביטול זמן החול and replacing it with holy time. הבדלה is the stoppage of this unique time.
[עפ"י תורת ראש הישיבה זצ"ל]
