Your argument only carries weight if you have some evidence that He has given Satan only one chance. The question for you is: How many chances has He given Satan, and how do you know that? Perhaps, since God is all-merciful and all-forgiving, He has given Satan many chances, and yet Satan willfully stays rebellious.

God can't be a all merciful all forgiving god if he is not willing to give everyone a second chance.

We find out from the Bible that God's mercy and forgiveness is unlimited for humans who repent, but that his mercy and forgiveness do not extend to the unrepentant. If you are interested in what the Bible teaches, that's it. If you want to speculate about how God's mercy and forgiveness extend into the spiritual realm, we are into such blind speculation that it's all guesswork and therefore nothing more than musing.

God's treatment of the Devil has nothing to do with His mercy and forgiveness towards humans. That's like speculating, "If you like all fruits, what would happen if we one day invented a new fruit that's not really a fruit?" It involves so much speculation in a completely different and unknown topic that a discussion takes us nowhere.

As far as for human beings, "where is the line drawn?" is a question I can answer. For human beings, the only sin that cannot be forgiven is the sin of continuing rebellion against God (habitual and unrepentant blasphemy). As far as Adolf Hitler, the poster child for evil, if he had truly repented before the end of his life and had a soul change, he would have been forgiven. But since we know there are degrees of reward in heaven, we can be assured that he would still be held accountable for the evil he had done (2 Cor. 5.10 et al.), and his reward would be greatly mitigated by his evil acts. He would still have been justified and forgiven, fundamentally, but that wouldn't compromise him being treated with justice (Rev. 20.12).

I disagree. The core of my question relates to the extent of God's mercy and forgiveness. What limit is there? If he is willing to forgive the Devil who is the complete antithesis of God and the personification of evil then God's mercy is truly infinite. If God wouldn't forgive him then there is a limit. The question then becomes for human beings where is that line drawn? If god wouldn't forgive the Devil then why would he forgive Hitler who is arguably the human personification of evil.

I'll admit that you've tried to post a cute little conundrum, but such speculation is just idle. 1 Timothy 1.4 warns against wasting time with such things because they don't take us anywhere valuable. We know from prophesy in the Bible that it isn't going to happen. Revelation shows us quite clearly that the Devil stays rebellious until the bitter end where he is judged by God and sent to hell, which was actually prepared for him, and not for people. The Bible speaks of forgiveness as a possibility for humans, but never discusses the matter pertaining to spirit beings. Such hypothesizing about Satan repenting doesn't and can't take us anywhere.

If the Devil repented and asked for forgiveness, would God grant him such a request? If not, why not?