Why did the Torah not give detailed instructions as to how to erect the Mishkan (in the way that he did subsequently erect it - starting from Pasuk 18)?
Ramban: Because Hashem had already shown him exactly how it was to be done. 1 And it is by the same token that the Torah is brief concerning the instructions regarding the Shulchan and the Menorah 2 and the Begadim of Aharon and his sons in Pesukim 13 and 14. 3
Why does the Torah write that Moshe will set up the Mishkan on Rosh Chodesh Nisan (the eighth day of the Milu'im), when in fact, he first set it up on the first day of the Milu'im, seven days earlier?
Ramban, Moshav Zekenim: What the Pasuk means is that on Rosh Chodesh Nisan, Moshe will erect the Mishkan for the last time. 1 It will stay erect until they travel, and then the Leviyim will dismantle it and set it up.
In fact, Hashem had instructed him to set it up (See 26:30), as soon as it was completed, and he extrapolated from this Pasuk that when he would set it up prior to Rosh Chodesh Nisan, it would not be on a permanent basis, but that he would have to take it down again each morning (some opinions say twice or even three times a day [See Ramban DH 'be'Yom ha'Chodesh' and 'u'Kevar Darshu']).
Why was Moshe required to dismantle the Mishkan each morning (Ramban) before putting it up, each day during the seven days of Milu'im?
Ramban: Either to teach the Levi'im how it had to be done, or in order to crown Rosh Chodesh with the crown of the inauguration. 1
See opening Rashi in Shemini. See also Ramban for a number of reasons as to why the Mishkan had to remain intact throughout the day and the following night, and could only be dismantled first thing in the morning.