Why did Moshe place the Mitzvah of Shemiras Shabbos before Meleches ha'Mishkan?
Rashi, Ramban and Seforno: To teach them that building the Mishkan does not override Shabbos. 1
Because the sanctity of time is greater than that of location (EC). See also Ba'al ha'Turim.
Having taught in Shemos (31:14) that Chilul Shabbos is subject to the death-penalty, why does the Torah see fit to repeat it here?
Seforno: To teach us that he is Chayav Misah even though he was busy with a Mitzvah.
Why does it say "Te'aseh Melachah" (passive)?
Moshav Zekenim: When Yisrael merits, our work is done through others.
Rashi writes that here the Torah teaches that building the Mishkan does not override Shabbos. Why is this needed? Shabbos is an Aseh and Lo Sa'aseh. An Aseh does not override an Aseh and Lo Sa'aseh (Bava Metzi'a 30a).
Tosfos (Yevamos 6a DH Nigamer): (Normally an Aseh does not override an Aseh and Lo Sa'aseh, for one Aseh no stronger than another.) Building the Mikdash is stronger than the Aseh of Shabbos, due to fear of the Mikdash.
Ritva (Yevamos 6a, citing R. Yonah): The Aseh of Shabbos applies only to Av Melachos that are stringent. 1
One might have thought that Binyan ha'Mishkan overrides Toldos, or making animals work
Rashi writes that here the Torah teaches that building the Mishkan does not override Shabbos. We already know this from (Vayikra 19:30) "Es Shabsosai Tishmoru u'Makdishi Tira'u" (Yevamos 6a)!
Moshav Zekenim: Our verse teaches that all the Melachos used to make the Mishkan are forbidden on Shabbos.
Why does it say here Kodesh before Shabbos Shabason, and above (16:23) it says "Shabason Shabbos Kodesh"?
Moshav Zekenim (16:23): This teaches that one must add from Chol onto Kodesh. Shevisah (refraining from Melachah) before the actual Kodesh, and also after the Kodesh (on Motza'ei Shabbos).