> In other words, he can't do what is logically impossible, with which I agree. Is this a reasonable interpretation of what you're saying?
Yes, that's fair, but in addition I'm saying that reality is the way things are and so God conforms his behavior to reality also, by necessity. If death is the consequence of sin, God just can't spin a fidget spinner to make it go away. He has to undo death by reversing the flow of how death came into the system.
> My disagreement lies in the fact that God harming himself is necessary for salvation, or a better world. I would request some expansion on this.
God is life; union with him is life, separation from him is death. (Like picking a wildflower. As soon as you pick it, you have broken its contact with life. It is now dead. The question is merely how long it will take to fade. Though it looks alive [for the time being], that won't last long. Though it still has nutrients running through its system, it is technically dead [separated from life] and fading.) Let's use that analogy (though that analogy has some flaws).
Adam and Eve were brought into the Garden and given access to the Tree of Life. There they could enjoy unity with God and be alive. God wanted them to have a relationship of love with him, and since love must always be chosen, not forced, He gave them a choice. They chose to rebel against him and chose to be separated from His life. Therefore that day they "surely died", meaning they were separated from God. All children born to them were also under the status of separation from God (sort of as if [another analogy] they had left their country to become citizens of another country. Their children were now citizens of the far country as well).
The Bible says that sin brought separation from God by choice, nature, and behavior. The separation was complete, and the result was "death" (separation from the life of God). The idea behind Penal Substitutionary Atonement is that someone who did not deserve to die took the punishment of death (archetypally) for all humanity, making it possible for them to be restored. The one who had never sinned became sin for us so that we might become life once again through his death nd resurrection.
It's not substitution in the sense of quantifiable exchange, but in the sense of a legal demand: Jesus, who didn't deserve to die, died as a substitution for those who did deserve to die. Somebody else who didn't owe the creditor anything paid your debt for you (death) and set you free.
That's the premise behind it. It addresses the problem because it relates to death and life, redemption and substitution. The only way to break the power of death is not to wave a magic wand, but to enter death and prove that it is not strong enough to hold you. (The only way to show a prison is not escape-proof is to enter the jail and prove that you can break out.) Otherwise it's just bravado: "I could break outta there." "Nuh-uh." "Uh-huh." "No you couldn't." "Yes I could." Somebody finally has to put their money where their mouth is and show that death doesn't have the ultimate power. Therefore the way to break the power of death and sin is to die as anyone else would, and then break out, to show that death has no power over you (Heb. 2.14-15).
> This depends on your definition of fair.
My definition of fair is where the punishment fits the crime, always appropriate and never a square peg in a round hole, and always appropriate and never too much or too little for the circumstances or person. "Fair" takes into account motive, mental state, information, opportunity, environment, attitudes, and intentionality.
> I am a staunch opponent of divine command theory.
I am, too.
> God should be swayed objective morality rather than simply making up his own rules.
God neither makes up his own rules, nor is He swayed. Objective morality is part of reality: It is what it is, but it is what it is because of the nature and attributes of God. In other words, it truly is objective morality. God didn't create it, He is it. Something isn't right because He commands it; it is right because that is His nature and it can be no other way. And if it's truly objective morality, then it is an uncompromisable standard of true rightness.
> I should also mention that burning people in fire is an unconstructive punishment that only makes more wounds rather than healing the wounds that perpetrators made on earth.
We are not to think of hell as fire (it's a word picture), nor are we to think of it as "One Fire Fits All." The Bible tells us that whatever punishment awaits people will be fitting to what they deserve (2 Cor. 5.10; Rev. 20.12), will be mitigated by degrees to fit their particular individual situation (Matthew 11.22-24; 23.14; Luke 10.12; 12.47-48; Revelation 20.13), and that God knows everything and will be perfectly fair with people.