But the overarching point still seems correct. The events in question unfolded over several hours, and it’s quite possible that the repentant thief began this time on the cross with derision, but ended it with repentance. There was plenty of time for both blasphemy and repentance. And as death drew closer, there was plenty of reason for repentance.

On the one hand, you’re right to point out that Luke uses the imperfect tense to describe the bandit blaspheming, thus suggesting continuing blasphemy over time (the NRSV’s “kept deriding” is intended to capture this better than “derided” would). That gives the second bandit a chance to join him in blaspheming initially (as per Matthew and Mark), but then repent and rebuke him later (as per Luke).

On the other hand, both Matthew and Mark also use the imperfect tense to describe both bandits blaspheming, thus suggesting continuing blasphemy over time from the second bandit too. The harmonisation therefore has to have both bandits mocking Jesus over a period of time and then the second repenting and rebuking the first, which doesn’t strike me as materially different from having both bandits mocking Jesus just once and then the second repenting and rebuking the first.

So although the use of the imperfect tense is helpful in one respect (by extending the blasphemy of the first bandit, it gives the second more time to repent), it’s unhelpful in another respect (by extending the blasphemy of the second bandit, it gives him less time to repent). These effects seem to me to cancel each other out; I don’t think that the tense makes much difference here.

Luke does say that one “kept deriding him…but the other rebuked him”. He doesn’t come out and say it, but there’s a hint here that the second bandit relented. In fairness, this ‘hint’ only appears in the NRSV and there’s no “kept” in the Greek, so that’s something you’d have to tease out of the tense/voice/mood of the verb “blasphemo” (which BTW is Imperfect, Active, Indicative).

Still, the harmonization would be more satisfactory if you had a reason for the change of heart. But the grim reality of death might be reason enough. Is it that hard to believe that both criminals might have started out mocking him, but that one, as the reality of his death began to sink in, changed his mind and his words?

There is a second, and far more speculative, possible reason for the second thief’s change of heart.

The next thing Luke says is that it was the sixth hour and the strange darkness fell. The NASB renders his words as “It was now about the sixth hour…”. (Your preferred ESV renders it the same way, and Errancy’s NRSV is similar, though it says, as we know all too well, “about noon”) That could suggest that this happened just as the previous action was taking place. If so, then the second thief may also have changed his tune because he connected the darkness with Christ and realized who he was.

Of course, “It was now…” could also mean right after the previous action, that’s why this second hypothesis is highly speculative, so I don’t want to make too much of it.