Was it the act of seducing Eve to eat the apple that was his downfall, was he already fallen before that, or was it after?
That happened before the Garden of Eden incident. Before then, he walked in heaven (on the holy mount of God).
14 You were anointed as a guardian cherub, for so I ordained you. You were on the holy mount of God; you walked among the fiery stones. 15 You were blameless in your ways from the day you were created till wickedness was found in you.
He was actually cast down because he became proud.
Your heart became proud on account of your beauty, and you corrupted your wisdom because of your splendor. So I threw you to the earth; I made a spectacle of you before kings.
Then he thought that he could be God, that he could take God's place in the universe as ruler of it:
13 You said in your heart, “I will ascend to the heavens; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon. 14 I will ascend above the tops of the clouds; I will make myself like the Most High.”
That was his downfal: Pride--he thought he was as good as God.