Since the entire reality of Shabbos is Olam Haba and the work of Olam Hazeh is a contradiction to this reality, it is therefore forbidden to work. But when it comes to building the Beis Hamikdash which is about the dwelling of the שכינה and Olam Haba, maybe things are different and it is permitted to work.
On the other hand, we can say that even if פיקוח נפש overrides other mitzvos it won't override Shabbos. That is because all the other mitzvos are part of this world and it can't be that something from this world will cost a person his life in this world. But Shabbos is from the next world so maybe one should keep Shabbos and can't violate it even if it will cost him his life. Similarly we can say regarding מילה and עבודה overriding Shabbos, since they are not direct melachos to bring the שכינה but rather simple mitzva acts. On the surface, we can't override the reality of Olam Haba for mitzvos of Olam Hazeh. Therefore it says "אך" as an exclusion and limitation vis a vis Shabbos, that despite the fact that Shabbos is really a taste of Olam Haba nevertheless that does not impact one's practical behavior where we say that Shabbos is like every other mitzva which is defined within the framework of this world. [So Shabbos is being limited and constricted in its Olam Haba aspect].
That is the meaning of the pasuk that says "דבר אל בני ישראל לאמר" - the word "דיבור" denotes לשון קשה, strict words, because this is a commandment that must be followed. No two ways about it. Then the pasuk continushould be enoughes "כי אות היא ביני וביניכם" - "because it is a sign between me and you", to say that it is a mitzva like all mitzvos. But it also says לאמר which is a לשון רך - soft words. In relation to this softness the pasuk says "לדעת כי אני ה' מקדשכם" - "to know that I Hashem sanctify you", meaning in Olam Haba as the Mechilta taught. It is an act of love from Hashem that the Jewish people can taste Olam Haba right here in Olam Hazeh. Then the pasuk says "'אך את וכו" meaning that the למעשה, practical fulfillment of this mitzva is like all mitzvos [and therefore certain mitzvos can override Shabbos as they would other mitzvos. And in the case of building the Beis Hamikdash, the laws of Shabbos remain in force as they would for any other mitzva despite our hava amina to the contrary as we explained earlier].
According to this, when it says "ואתה תדבר" it means that Moshe specifically should give them the mitzva of Shabbos בלשון קשה. For as we said the לאמור, sweet Olam Haba aspect of the mitzva doesn't impact its practical fulfillment. Moreover, the Mechilta says
ואתה תדבר - אבל לא ע"י מלאך ולא ע"י שליח.
YOU should speak but not an angel or messenger.
The Malbim explains that since Shabbos was already given in the Aseres Hadibros it should be enough if Aharon gives it this time [see Eiruvin 54 "כך סדר משנה" that Aharon taught the Jews Torah]. That is why we need the words "ואתה תדבר" to teach that specifically Moshe should give it. Based on what we explained this means that if it was just teaching is the details of the mitzva such as whether it overrides פיקוח נפש or the like, then it would be enough for Aharon to give it. But in fact we are learning the fundamental definition and nature of the mitzva. "אך" teaches us that it is a mitzva in the same category as all mitzvos [despite its Olam Haba aspect] and therefore as a result does not override פיקוח נפש and עבודה. Since we are dealing with the basic definition and nature of the mitzva we need Moshe to teach it.
[עפ"י תורת מו"ר הגאון הגדול רבי ד"י מן זצ"ל]
