This thief was no dummy. He was actually quite astute theologically. No doubt he had seen and heard Jesus somewhere along the way, but had never repented. He was fully aware that Jesus was innocent, and had been crucified because of the hatred and malice of the Jewish religious leaders and the rabid mob.
But he also noted the inscription above Christ while on the cross, declaring Him to be King of the Jews. And then he noted both the grace and majesty of Christ, who did not murmur or complain, but hung there with dignity, and forgave His enemies. He may have overheard the instructions that Jesus gave to John regarding His mother.
Therefore he came to the realization that Jesus was truly the Messiah and the King of the Jews, and he rebuked the other malefactor for reviling Christ. Finally he called Him "Lord" and confirmed that he believed in Christ's future Kingdom on earth (unlike many Christians today). Therefore he asked to be remembered in that Kingdom.
Although, due to the lack of evidence, I question entirely your predication that the malefactor was astute theologically, but rather, possibly astute spiritually. That is, like you pointed out, there was enough evidence within the immediate context of the crucifixion, that anyone with a predisposition to wisdom, might have recognized the injustice of the charge. For example, the vehemence of the mob in contrast to Christ's grace and dignity, ...and the other points that you mentioned. For even Pilate perceived the deception on the Jews part, and the Roman soldier at the cross concluded that Christ was the Messiah.
I may hypothesis that the two criminals were present at Christ's trial, as they awaited their previously sentenced fate. And thus, witnessed the farcical and ungrounded charges at the proceedings? Assuming that during a crucifixion, all those condemned are gathered together at a certain point, somewhere between the judgement courts and the place of crucifixion. Again, mere speculation, because it may not really explain the sudden turn-around from the mocking to the praising from the thieves, when both dispositions took place while exclusively on the cross?
Either way Enoch111, your brought up some very perceptive points, and I personally think that they are the most viable of what I've read so far on the thread.