What exactly, did Moshe transcribe?
Ramban (in Pasuk 1), Rashbam and Seforno: Refer to 24:3:1:2.
R. Bachye: It refers to the conditions mentioned in Yisro (19:5) ""Vehayah Im Shamo's Tishme'u be'Koli u'Shemartem es B'risi".
The transcript referred to here is synonymous with the "Seifer ha'Beris" in Pasuk 7.
When did Moshe arise and build a Mizbe'ach?
Rashi: On the morning of the fifth of Sivan. 1
Ramban: On the morning of the seventh of Sivan. 2
What is the significance of the twelve Matzeivos that Moshe built?
Rashbam: It was a testimony that all twelve tribes agreed to enter into the covenant with Hashem.
Refer to 15:27:1:1*.
How could Moshe build Matzeivos, when, as Chazal stated, after the time of the Avos, the Matzeivah became hated in the eyes of Hashem?
Oznayim la'Torah: That was only because the Nochrim adopted the Matzeivos for idol-worship, 1 and as long as Yisrael were in the desert they had not seen it and would therefore not be drawn after Avodah-Zarah.
As Rashi explains in Parshas Shoftim, Devarim 16:22.
