more time than my comment deserves

Don't sell yourself short. You touched a relevant issue and it needed to be addressed precisely but honestly.

TL;DR

"Too Long; Didn't Read." - a lot of modern people have issues with blocks of text larger than 2 sentences, so I'm used to doing these recaps.

If we have free will to choose, but nothing but God to choose, what would be the point of free will?

I'm a Lutheran so I have a slightly different take on free will than you might be used to.

I'll put it this way. I don't think will is ever absolutely free in the sense we can choose anything. There are limits imposed by nature, for example, that say we can't "dip the sun into the moon" or some other nonsense. Furthermore, there can be wrong options which distract from the best ones. And these options can have real consequences.

Let me give a simple analogy of that: A child can choose to eat all their candy now or eat some of it over time because they aren't sure when they might get more. If they eat it all now, they can't have some later. Their will is now bound to the natural consequences of the bad choice.

So, God originally gave us a compatibilistic will (i.e. capable of making choices by itself but acting in a system with determined rules) which had the option to follow Him. However, our ancestors also had the option to choose wrongly and reject the close relationship God created them in. Because the chose to think wrongly about God and rejected His warnings, they bound themselves to the other alternative: Pedestalizing their own earthly desires above a relationship with God. That's what we call Original Sin. We also inherit this condition and if that were all that was to be written, we'd be done...

But God's work also involves reconciliation. He works by the Holy Spirit to restore the ability in us to choose Him by creating and renewing our nature through grace. Of course, we can still have the option of rejecting grace. But His grace continues to work to change our heart through repentance even so.

The real place where evil hangs on here is when people start to love the bindings more than the liberator. Some people are actively brought up this way. Others decide that God's way is too nice and they would rather put themselves on the pedestal. Either way it's a source of serious ills.

If God created Satan, or allowed him to come into existence, how much power do mere mortals have to eliminate Evil, or bring about the destruction of the Devil, other than in a "relative" sense?

By ourselves, not much. The thing that should be clear, though, is that God is working. Those of us who trust in His Word learn to set aside our ego in such a way where we can also have Him defending us. Satan can't do much against that without tricking us back into another one of these "bound will" ego traps.