Yes, grace IS the opposite of works.
Question: Was God ever a gracious Host to Lucifer and angels-turned-demons in heaven? (If so, how then was God's role as a gracious Host ever revoked -- if what you say was "so"?)
You can’t “fall from Grace” when you have been truly saved by God (not of yourself).
Hebrews 6:4-6: 4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the powers of the coming age 6 and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Interesting question(s), while I don’t know for sure I believe Satan’s sin was so well it was done in much greater knowledge (and turning from God) than our natural death as sinners...however I cannot really answer fully!
Second question. I would say that they may have known the truth, and been drawn by the Holy Spirit, but weren’t really saved. A very dangerous place indeed, as they will never be saved!!