The introduction of Michael at this point is relevant because, according to the Enoch legend, it was none other than Michael who was the leader in bringing the accusation against the fallen angels to God:
Thou hast made all things, and power over all things hast Thou: and all things are naked and open in Thy sight, and Thou seest all things, and nothing can hide itself from Thee. Thou seest what Azazel hath done, who hath taught all unrighteousness on earth and revealed the eternal secrets which were preserved in heaven, which men were striving to learn: And Shemihazah, to whom Thou hast given authority to bear rule over his associates. And they have gone to the daughters of men upon the earth, and have slept with the women, and have defiled themselves, and revealed to them all kinds of sins. And the women have borne giants, and the whole earth has thereby been filled with blood and unrighteousness" (I En. 9:1-10, translated by R.H. Charles, 1912).
So, according to I Enoch 9:1-10, it was Michael who accused Shemihazah and Azazel, but according to Jude, Michael "would not dare to bring a slanderous accusation," even against the devil himself. In other words, the story of Michael making an accusation against the angels in Enoch is false, and if the story of the accusation is false then so is the story of the angels’ sin.
The above explains why Jude chose to substitute "Michael" for Peter’s more general "angels," but it doesn’t explain the mention of the devil and the body of Moses. The devil, Satan, does not appear in Enoch, and cannot be identified with Shemihazah and Azazel, the leaders of the 200 rebel angels. Therefore there must be another reference to Michael and the devil elsewhere.
The remaining two possible sources are:
2 - The Assumption of Moses
3 - Zechariah 3’