Matthew says both thieves ridiculed Jesus. Mark and John do not mention the thieves saying anything. Luke says that one thief, by his comments, evidenced that he finally recognized Jesus for who he was and defended him, but that defense came after he believed. It wasn't the works that saved, but the belief. The justice of God demands that all sinners be treated the same way as far as salvation is concerned. If there are exceptions, even one, then God is capricious, not just. It leaves open the argument that those who never heard the gospel, or culturally disbelieved because of nurture should have another chance or have an exception to the plan of salvation thus devaluing the death of Jesus.
“The Calvinist approach I’ve seen here is that of some shabby lawyer looking for a loophole in some shabby rental agreement.”
Insult is an awkward homage which inferiority renders to merit. If you don't have the facts in your favor, use ridicule and insult.
I'll keep that in mind if I'm ever in that position.
Such argument indeed exists, it is called invincible ignorance. It does follow from the scripture, as you astutely note. But why, do you think, does it devalue the work of Jesus? If it were not for His Passion, no one at all would be saved.
Wow. That's good. May I steal it?