My current understanding is that each person has existed eternally as an autonomous (but not, initially, personal) essence - and that at some point in Time we became Sons of God, which made us into persons.
(God shaped us into personhood when we became his spirit children, before we entered mortal life.)
Our free will is rooted in our eternal autonomous existence, but was made effectual - choices were made possible - by our having become Children of God.
This is what made The Fall possible.
On the one hand, our personhood comes from God and the reality of our situation is that we are in a profound relationship with God since He is our Father and made us persons; but on the other hand we existed as essences before we had a relationship with God; and this pre-existence is what enables us to reject God, and to deny the primacy of our relationship with Him.
It is because our free will derives from eternal agency that we are able to choose (to have the divine attribute of being unmoved movers, or first causes).
And it is because our free will derives from eternal agency that we must choose to acknowledge God's Fatherly love for us, and our child-like love of Him - because we cannot be compelled (not even by God - it is vital to recognize this) to acknowledge God's love, nor can we be compelled to love Him.
To be Christian is a choice because it must be a choice.
Therefore, Satan could not and cannot remove the ultimate (metaphysical) autonomy of persons, nor can Satan control free will - although he can of course enslave the body and compel actions.
Satan can influence autonomy only indirectly - principally by demoralization and corruption of the will - so that a person will choose to use his autonomy to deny his autonomy; and deliberately, repeatedly, systematically choose to sin and to destroy Good - while denying at every moment that he could choose otherwise.
And this is, of course, the great triumph of Satan in this modern era: to have so deeply confused and corrupted modern man that he uses his eternal and indestructible freedom of will in actively-denying the reality of his own freedom.
[Note: The above schema is the only one that (currently) seems to make sense of free will to me, therefore I present it for consideration. It leaves intact all core Christian doctrine, but modifies the metaphysical back story - so that some things are re-explained. It is, I think, pretty much identical with the implications of Mormon theology as I get it from Sterling McMurrin and Terryl Givens - but there are quite likely aspects which go beyond, or conflict with, what many or most Mormon theologians seem to say - so far as I can tell - which is not very far.]