You're right. To redeem all of creation it requires that the fallen spirits be redeemed as well. And you're right again, there is no single verse (to my knowledge) that says the demons will be forgiven.

But there are passages that imply it will happen and I ask you to carefully consider those:

1. 1 Corinthians 15:20-28. This passage is the locus classicus for what the final end state of things will be. Paul says here that once Christ has overcome every enemy (which surely includes Satan and his demons), placing them in subjection to himself, He will offer up creation to God who will then be all in all—or, as some translations have it, everything to everyone. It is hard to see how God can be ‘all in all’ if there are still holdouts. So I don’t think Paul leaves open the possibility of unreconciled evil spirits. God will someday be ‘everything’ to them and no part of creation will be out of harmony with God.
2. Romans 14:11 and Philippians 2:9-11. The key word here is ἐξομολογέω, which can mean ‘to confess', as in to acknowledge one’s sins, but it can also mean ‘to joyfully praise’. Here again Paul is looking toward the final state of things and, I would argue, Paul chose this word precisely because it gives an accurate picture of what will happen. Satan and the demons will confess their sins—but it will be through the very act of praising Jesus. You cannot call Jesus Lord and Savior and God without simultaneously humbling yourself, admitting your need, and turning to Him, which is exactly what the fallen spirits will do. And to those who argue that this will be a forced confession and forced praise, I return that a confession may be forced but praise cannot be. Praise, by its very nature, must be genuine. Another reason why I think Paul chose this word.
3. All the verses that say Jesus is the savior of the world. We narrowly think of the 'world' as just a small subset of human beings. But κόσμος is much more than that. It means everything, every single thing that exists—and also how it exists, the way it functions, the order of things. Jesus came to do much more than save a few people. He came to remake creation, make everything new, put things in their proper place, and give them their proper function. If you approach Scripture with this broader purpose in mind, redemption, including the demons and everything else—plants, animals, rocks, even the entire universe as it slides toward entropy—is much easier to see.

So yes, Satan and the demons will be ‘forgiven’—although a better way of putting it would be to say they will be purified to the uttermost farthing and afterwards reconciled, same as any other sinner.

Here are a couple of essays I think you will enjoy.

• In illud: Tunc et ipse filius
• On the Question of the Apocatastasis of the Fallen Spirits

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