What purpose did the cover of Tachash-skin, which covered all the holy vessels, serve?
Ramban: To protect the holy vessels from the rain. 1
One wonders why the Pillar of Cloud, which protected them on all sides and covered them on top when they traveled, did not protect them from the rain?
Why, in contrast to all the other holy vessels, was the Tachash-skin covered with a cover of Techeiles?
Ramban #1: In deference to the holy Aron, it was covered first by the Paroches (its own curtain), then by the Tachash-skin, to protect it from the rain, and on top, by a cover of Techeiles - that one should see an honored garment of Techeiles, that resembled 'the very heaven in purity'.
Ramban (citing Ibn Ezra): The Pasuk means that they placed the cover of Techeiles on top of the Aron and the Paroches, and a cover of Tachash-skin on top (of that).
What is the meaning of "v'Samu Badav"?
Ibn Ezra #1, Ramban #1, Moshav Zekenim #1: It means that they placed the staves on the shoulders of those who carried it. 1
Ramban #2, Moshav Zekenim #2: It means that they arranged the staves in the rings in order to transport the Aron. 2 Da'as Zekenim (5)
Our text of the Ramban says "the Kohanim's shoulders." Even though he holds that also Kohanim carried it, it seems that this is a printing mistake. (R. Chavel's footnotes).
Kesef Mezukak (in R. Chavel's footnotes): When the Mishkan was erected, the poles extended to the Paroches. When they carried it, they were half in front and half in back.
Why does it say "Es Badav" regarding the Shulchan and the gold Mizbe'ach, but not regarding the Aron and Mizbach ha'Olah?
Moshav Zekenim citing R. Yehudah ha'Chasid: In the Aron and Mizbach ha'Olah, the plating (Batim) on the poles was fixed in the rings. They inserted [or fixed] the poles themselves, without the Batim. In the Shulchan and the gold Mizbe'ach, the Batim were on the poles. "Es" teaches that they inserted the poles with the Batim. 1
Moshav Zekenim: What was Betzalel's source to make Batim? He was not commanded in Parshas Terumah! At the time of the command, it says "Es"(regarding the poles of all of them) to include the Batim. "Es" is not repeated [regarding the poles of the Aron and Mizbach ha'Olah], for they were left permanently in the rings.