Here’s a Question that my son got on his mission about the war in heaven: “If the argument in heaven was over who would be the Savior, and that even led to Lucifer becoming Satan, then what was the Savior going to save us all from in the first place because technically Satan hadn’t happened yet.”
Two points.
First point — the war in heaven
The poser of that question is saying that Lucifer only became Satan after he gave Eve the fruit of the tree of knowledge. But Lucifer became Satan before the Fall. He rebelled from God in the heavens, started a war in heaven and was tossed out on his ear, along with his followers down to earth. Satan was on earth when Adam and Eve arrived in the garden. He was already Satan.
Mormons believe that we had a life before this one on earth, where we lived as spirits, with our God, in a sphere where we were learning and progressing. As a good Father, our God wanted to give us more opportunities to progress — to become like him, with an immortal  body of flesh and eternal life.
He wanted to give us the chance to do as He had done. (Abraham 3: 22-25) Â
In this pre-earth life, God met in council, planning for our future progression — an earth where we would exercise our agency in choosing good or evil, experiencing opposition in all things.
Lucifer was at this council as well,  and he offered his plan — he wanted to take away our agency — and force everyone to keep his commandments.  He wanted God’s glory and power for doing this as well. “I will redeem all mankind that one soul shall not be lost…wherefore give me thine honor.” (Moses 4:1-4)  Lucifer wanted to take away our agency — he didn’t want us to progress.
Obviously, by taking away our choice, and compelling everyone to do what is right, no one would sin, and there would be no need for a savior — which may sound ideal, but we would have lost our agency and ability to progress and become like God.
God’s plan was to provide us with an opportunity to advance, use our agency to build faith, to become like him — which necessitated a fall and a savior. Jesus was willing to take that role, to save us from our sins when we fell. He made the sacrifice because of his great love for us and God’s plan that would enable us to become exalted.
Two plans were on the table — Lucifer’s and God’s. This was the war in heaven. Most of us in the spirit world wanted our agency and chose God’s plan. Michael and his angels fought against the Devil and his angels — the result being that Satan and his minions were cast out to earth (Revelation 12:4, 7-9). One-third followed Lucifer and they were cast down to earth.
How art thou fallen from heaven, O Lucifer, son of the morning! For thou hast said in thine heart, “I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the amost High.”  (Isaiah 14:12)
Satan and his minions wander around here as evil spirits and still try to prevent our progression. They try to get us to make bad choices, because they glory in our failures. The war in heaven was about agency and progression and it continues here on earth.
Lucifer became Satan before the Fall, while in our pre-earth life, when God and 2/3 of us did not accept his proposal to force everyone to keep the commandments, lose our agency and be subject to Lucifer. It was then that he took his 1/3 followers and started the war in heaven, fighting against God and Michael, until he was banished to earth. And that’s where the rest of the story picks up — Satan enters the garden in hopes of derailing Gods plan for our progression. But Satan did not know the mind of God, and when Eve took the fruit, she was fulfilling her calling to open the way for all of us to become like God.
Second point — the Fall
That question assumes that God did not have a plan for us that included the “Fall.” Did God intend Adam and Eve to remain in the garden forever? And have no need for a savior? Was God unknowing, and surprised when Satan showed up and gave Eve the fruit? Were Adam and Eve to blame for our situation here? And did God have to scramble for a solution to the problem?
While many believe this to be true, I don’t.
What was the Savior going to save us from since the fall had not happened yet?
He was going to save us from the effects of the fall when it happened. Yep, it was all part of the plan. The plan included the fall. It was not a mistake. It was expected that Adam and Eve would choose to fall, by partaking of the tree of knowledge. A savior had to be chosen to ensure their return to God. No savior, no return. Earth life, the fall, would bring sin into the world, an unavoidable effect of partaking of the tree of knowledge.
The plan was for us to fall. The fall gives us an opportunity to rise higher than we were before our mortal  birth.
Most Christians believe God created us at birth (we had no pre-existence) and that Eve made a huge mistake when she took the fruit. And she and Adam brought the fall when it was not needed or wanted. They think that God had to fix the problem and bring a savior to save them. They don’t believe that we can become like God and that there is a purpose for the Fall. They don’t believe that as man is God once was. They don’t believe that it was a blessing when Eve partook of the fruit, so we could have a chance to progress.
I believe Eve knew what she was doing, it’s just that the serpent deceived her about it. She may have thought the serpent was a messenger from God that was giving her the fruit, (the word translated serpent is a hebrew word meaning shiny one.) She knew she had to go forward if she wanted to learn the difference between good and evil, and have progeny and become like god. All the gods before us have done it this way. It’s the only way. We have to gain knowledge of good and evil by experience and choice.
We must fall to progress.
When Adam and Eve ate the fruit of the tree of knowledge,  the Lord God said, Behold, the man is become as one of us, to know good and evil. (Genesis 3: 22).
That was part of the plan. As Eve asked, “is there no other way?” — There is no other way.
(Hugh Nibley offers insight on ancient teachings about the council in heaven, see Lecture 17 of Teachings of The Pearl of Great Price Lectures .)