There seem to be some things you are missing about this subject of God's character. In spite of God's mercy to Cain as pertains to this life, I doubt poor Cain has inherited eternal life (see Jude).
That's not the point of forgiveness. Poeple want God to overlook their sins...well He already does that. But sin is a SYMPTON of a bigger spiritual problem, that you are not addressing...or your cohorts are addressing.
When a leper is healed...is the leprosy healed or just the skin cleared up temporarily?
When leprosy is healed the root of the problem is healed. Such is the gospel...it heals us from the disease of sin...the affliction. the bondage of sin.
Forgiveness is an understanding of one's condition...but not the solution to it.
The most foolish notion possible is to believe Jesus came all this way just to forgive sins. In fact it is a lie. Jesus came to set the captives free. Forgive a man his sin, and he will continue to sin. Free a man from his sin nature and he will not need forgiveness again.
And in spite of the rain mercifully falling on the just and unjust alike, it's pretty certain we will not see the unjust in heaven one day.
Do you mean Valhalla? Or the New Earth?
I'm quite certain that we won't see many who name and claim salvation foolishly.
Even the righteous Jews under the Law could only be justified if they had FAITH.
Not so. The righteous DO what is right. The peculiarity of the righteousness of faith is that it is transportable into the New Covenant of walking in the perfection of holiness in Christ. No works can translate you into the heavenly walk. It is by faith.