Did Satan and his 1/3 part followers “sin” in choosing not to follow God’s plan? Or are only those who gain a body able to sin? We definitely had power to make choices in the pre-existence, but when Satan “fell,” did he become sinful?
The short answer to your first question is, yes, Satan and his followers sinned in rebelling against Heavenly Father’s plan. So the question is, how do we know this? In 2 Peter 2:4, Peter describes as sinners those who were cast out for rebellion: “…God spared not the angels that sinned, but cast them down to hell…”
Also, in 2 Nephi 9, Jacob describes the fate of those who rebelled (emphasis mine):
8 O the wisdom of God, his mercy and grace! For behold, if the flesh should rise no more our spirits must become subject to that angel who fell from before the presence of the Eternal God, and became the devil, to rise no more.
9 And our spirits must have become like unto him, and we become devils, angels to a devil, to be shut out from the presence of our God, and to remain with the father of lies, in misery, like unto himself; yea, to that being who beguiled our first parents, who transformeth himself nigh unto an angel of light, and stirreth up the children of men unto secret combinations of murder and all manner of secret works of darkness.
When Satan was cast out from the presence of God, that was a punishment. Punishment is a consequence of sin. In that same chapter, Jacob teaches that punishment is the consequence of disobeying law:
25 Wherefore, he has given a law; and where there is no law given there is no punishment; and where there is no punishment there is no condemnation; and where there is no condemnation the mercies of the Holy One of Israel have claim upon them, because of the atonement; for they are delivered by the power of him.
Had Satan and his followers not sinned, they would not have been punished. But let’s be perfectly clear about it: Satan and his followers did a lot more than simply choosing not to follow God’s plan themselves. The scriptures teach us that Satan did much more than just make a choice:
In Moses 4:3, we learn that “Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power”. In Revelation 12:7-9, we learn that Satan waged a war against those who chose to follow God’s plan. And, in Doctrine and Covenants 93:25, we learn that Satan was a liar from the beginning. All these things are clearly against the will of God, and to act against God’s will is sin.
Satan and those who succumb to his influence may try to persuade us that there was something else at play here, but the scriptures make it clear that Satan used his own agency to rebel to the point of war. He still wages that war today. Let us not be lulled or deceived by his tactics, but seek always to learn and do the will of God so that one day we can return to live with our Father in Heaven, in perfect joy.