One important things seems to be missed... while we can't change God's mind and will, He can. Are we really qualified to hold Him to one will? We trust that God will stick to His promises, yes, but we are entirely relying on Him to do so, having no power ourselves to make a difference. So who are we to refuse God the freedom to change His mind (as if we could do so)?
In a way, whatever happens and has happened between God and Satan is their business, not ours. We get involved only where Satan drags us into it. But anyway, that aside...
This is, of course, unfounded, but I tend to think that Satan knew what he was getting into from the first moment he rebelled. Perhaps his pride was such that he was willing to spend eternity in hell as long as he could do what he wants for a while... sorta like a person who takes up smoking even though our society makes the detrimental, long-term side effects common knowledge. Of course, that example doesn't have so much to do with pride, but you know what I mean.
Anyways, what better way for Satan to go with his pride than to spite God, and what better way to spite God than to mess with His ultimate creation? Of course, God allows it because it's in keeping with His will (which I still say He can change if He wants to).
Oh, and Eon, where did you get this whacky idea that hell depends on fallen angels to function? It doesn't depend on them any more than any men that end up there. Now, if absolutely everyone were forgiven and hell were empty, then I suppose you could say it would shut down.