I don't understand why it would make a difference when someone received the testimony of Jesus as long as he received it eventually... or, at least before the Final Judgement.
Because it takes more faith to accept it the first time imho. But we do know that God has said this life is the time to prepare to meet God (Book of Mormon).
Why would God want to discriminate against those who didn't receive a testimony of Jesus during mortality?
He doesn't. That is why the gospel is preached to those who have passed on, so we can all be judged by the same standard (1 Peter 4:6). That same standard however is that if given the opportunity in this life and we reject it, one cannot have eternal life in the CK. However, if one never had opportunity in this life, then one is still eligible for eternal life in the CK if ones accepts it in the afterlife.
Is God somehow constrained by law from allowing these of His children to inherit the CK?
Seems to be some law, yes. I think it makes perfect sense. The scriptures teach us that we are here to see if we will obey all of God's commands (Abraham 3). As mentioned above, we also know that this life is the time to prepare to meet God, not later. So I do agree that it is the perfect compromise between justice and mercy to reward those who accept at first opportunity with a chance for eternal life.
Even Christ himself alludes to this in his parable of the Sower.
I mean, I just don't get it. Suppose that someone is a kind, generous, righteous Buddhist his whole life, but when the missionaries find him in his old age, he's set in his ways, and understandably isn't interested in reading the Book of Mormon or learning about Jesus.
I don't know what would happen. We LDS don't have a definition or any revelation of what an opportunity or a chance is. My opinion is that it's different for everyone and is as varied as their individual circumstances. But I would agree that once one recognizes the Spirit testifying so strongly, then perhaps one's first opportunity has come.
But then suppose that someone is a fornicating, live-in-girlfriend-abusing, drug-abusing, non-child-support-paying thief until he's about 30 or 40, and upon being released from prison, he meets the missionaries, sees the light, repents, and changes his ways. He gets baptized, sealed in the temple, etc. But he has hurt a lot of people. His children and victims have suffered greatly due to his misdeeds, and he can't really make it up to them. He hasn't lived nearly as virtuous a life as the Buddhist. Yet, he'll be eligible for the CK.
Are you familiar with the brother of the prodigal son in Jesus' parable of the same name? You are sounding just like him.
Seems pretty twisted to me. It also seems to directly contradict the parable of the laborers:
The parable of the laborers doesn't conflict in the least.