Which means that Satan was scoffing at God’s plan of salvation, scoffing at Jesus Christ’s role in the plan of salvation, and scoffing at the idea that there was going to be a Son of God who could redeem spirits sent to the earth who could through Him become sons of God and joint heirs.
Satan scoffs and ridicules at such an idea or such a plan. He is opposed to it.
"Going to be" a Son of God? Jesus is the Only true Son of God. He has always existed as such, long before anything else that exists was ever created. There was no “going to be” about it. Lucifer knew exactly Who Jesus was. He was jealous of the fact that when Jesus was incarnated as a man, all the angels would have to bow down to worship Him, and would all be subject to Him. But, Lucifer’s narcissistic pride in his great beauty, and in the majesty that he possessed as the highest angel in Heaven, would not allow him to accept that he would ever have to bow down to any ‘mortal man’, not even if that Man was God, Himself.
Lucifer knew that all the angels were created long before man, and even the lowest angel in Heaven had much greater gifts of intelligence, beauty and power than all mankind would ever have. He also knew that he was the greatest and most beautiful of all. So, he thought it was beneath his great dignity to ever worship and pay homage to any mortal man. That was what got him thrown out of Heaven, because he wanted to be worshiped by man, as a ‘god’, instead. He wanted all the power that only God possessed. In other words, he wanted to be equal to God. But, God refused to give him any more power than he already had.
That’s also why he wanted mankind to be punished by God, just like he was punished, by being thrown out of Paradise. He knew that if he could get them to sin, God would have to punish them. He was jealous of God’s love for mankind, and he hated them for it. What better way was there for him to instigate them to sin, than by tempting them with the same sin that he committed? He tempted them with the sin of pride, to want to be independent and not have to be obedient to God, but to have the same power that God possessed. To be His equal, instead of being His humble and obedient servant, who was content with all that God had already given to them. Satan tempted them to want even more than they already had, just like he did.
IMHO, Joseph Smith had absolutely no understanding of God, or the Bible, at all.