What would happen if Satan should decide to turn good and ask God to forgive him?
That’s up to God. We should never presume to know the mind of God because He is greater than and beyond all things. I believe God is unlimited. There’s no room for narrow-mindedness when ultimate power is given to God. HE is capable of the ultimate forgiveness because HE is God! Not even Michael the Archangel would condemn Satan. Only God has the right to judge Satan.
Actually, Satan is not necessary. Satan is just the impersonation of Evil, but Evil does not need an impersonation. By the way, God is often supposed to be the impersonation of Good, and Good does not need an impersonation either. But actually, it is a little more complicated, because God is supposed to be everything. Okay, it is not really traditional Christian theology…
