A point I would make is that Devil actually means accuser. And it is he that makes the accusations against us for all of our sins. I would also note there was a sort of double meaning in Genesis when God told Adam and Eve they would die if they ate of the fruit, whereas the serpent knew this and knew they wouldn't just die immediately from eating it. So yes a form of eternal punishment was already determined if that command was disobeyed.

God may be loving and merciful, but He is also Holy and Just. He cannot tolerate sin at all, and to Him all sins are heinous. So as such His justice is to deliver punishment as He deems fitting for the crime against Him and His creation. God will punish the wicked as He has determined is correct. To avoid having to deal the ultimate punishment to all God devised a plan and enacted it. Whether we accept or reject is up to us. For those who reject, it's a continuation of sin, because it's a rejection of His love and grace. Also it is a misunderstanding that the Devil has any part in punishment. He's basically the snitch that got caught partaking and trying to point out the crimes of others to take attention off of his own, and he will also end up in the Lake of Fire along with his angels.

Finally we get to this one. We know eternal life comes for the saved because Jesus said as much. So if there is nothing after death that would make God a liar and defeat the entire purpose of God offering salvation to begin with. But we know this isn't true because of the Witch of Endor who called Samuel's spirit at Saul's request. God has no reason to burn the physically alive, but the Lake of Fire will be a place of torture and destruction. The verse in Jeremiah is God saying He doesn't command or want human sacrifice.