I can’t say I know much about devils etc, but I tend towards the cosmic warfare view (a la Greg Boyd) that takes devils as fallen angels who have negatively influenced creation for a long, long time (I see the brutality of the natural order and evolution as pretty much lying at the feet of satan).
Where I differ from Boyd and others is that I believe that all creatures, including satan, will ultimately be reconciled.
God created the original order with the intention of having a good universe full of autonomous beings who freely chose to love him. This original creation contained an element of self-unfolding potential (non-deterministic physicality) and angels with power to influence, to a degree, the physical world - god works, as love works, by participation and empowerment of the other. Gods original plan remains his only plan, and he will succeed in this plan in the end.
But free will entails risk, and some angels rebelled. Whether they rebelled all at once or over time I do not know. The effects are clear to see. God has been working in the physical realm, again through participatory agents -angels and humans, to bring about his first plan, his kingdom.
How can god guarantee satans repentance? In the same way he guarantees all creatures will eventually choose him. At the start of creation god could have overwhelmed his beings with his presence, a full, tyrannical or coercive demonstration of his being which would have ensured obedience but removed freedom. So he withdrew and veiled himself to a degree, there was epistemical distance even between satan and god - the devil did not fully know the true nature of god or his love. But there is another way to guarantee obedience without loosing freedom - persuasion. If god could close the epistemical distance in a such a way as to provide a fully persuasive demonstration of his love he could provide a guaranteed way of having all creatures freely choosing him. The method of this epistemical collapse is the cross of Christ, it is in seeing the crucified god that a creature comprehends the truth of gods nature and turns to him. The more fully one understands the cross, the more one is turned to god. God will ensure that through the participatory action of angels and humans all remaining beings will eventually ‘see’ the beauty and truth of the cross, and repent.
This method of persuasive, not coercive, epistemical collapse was not available to god at creation for true love can only be fully revealed in sacrifice for ones enemies - a quality which could only exist in a world where free willed beings had already introduced evil and suffering. Thus by causing suffering and crucifying gods son the satan created the conditions that would eventually undo all his own evil - if they knew what they were doing they wouldn’t have crucified The Lord of glory. God works through redemption ( again, consider evolution - god turns death into a creative force that results in biological altruism, appreciation Of beauty, human reason etc) turning that which meant for evil into a means for good.
Thus god fulfills his plan - one united free universe of love - not only despite the evil of satan, but even due to it. God doesn’t will evil - free willed beings could always have chosen god from the start - but in creating evil, satan lay the foundation of his, and every other beings, eventual, guaranteed, irrevocable, freely chosen love of god through gods persuasive demonstration of goodness. God is very smart.