4: Maybe Quirrellmort doesn't have Voldemort's abusive impulses, because Horcrux!Harry is holding onto them.

I was thinking the same thing. It goes with the "make Harry the Dark Lord and then upload into him" theory. I'd spin it a little differently, though. It's not that he just tortures for fun, but that he is completely indifferent to the suffering of others. So torture is useful if it serves a witty joke, or gains him a nickel. It goes with Harry's intent to kill, and his "Heroic" consequentialist morality. His job is to "get the job done". Also, the demented Harry wasn't proposing to torture people for the glee of their pain, he was just proposing that the death of the annoyers would "get the job done" in removing annoyances.

It's unclear to me that any of the stories of Voldemort's "surplus evil", reveling in sadism, are necessarily true. They all happened offstage. Further, it's unclear that he was even totally indifferent to the suffering he caused. Just as I think Dumbledore took "credit" for burning Narcissa to seem more ruthless to his enemies, might not Voldemort have done similarly all along, to spread terror? That he was quite ruthless in waging war, I have little doubt. But the reports of surplus evil are just stories. Did Draco actually see Voldemort have Bellatrix crucio herself? Did anyone? Did Voldemort actually kill the dojo? How did they verify that the skin on the wall was actually of that guy? Even if it was, how does anyone know what order the skinning and killing took place in? Do we even know that the snake in the chamber of secrets is dead?

So maybe Voldemort hasn't changed at all as Quirrell.

Whether it's Lucius, Malfoy, or Voldemort, EY doesn't seem to deal in black and white. For any human, he doesn't. For Death, yes, but humans, no. No one is the bad guy in their own story.