i don't see why padme needs to know that anakin kills the sandpeople. it's very heavily implied by the action in the camp that he kills many of them, and unless my memory fails, we see him kill at least one. (i haven't been following your changes to the letter so perhaps you've changed this and I'm not aware of it.)
i think the anakin/padme scene in the garage is better without anakin discussing killing all the sandpeople. i don't see why it can't just be about his regret and sadness over the death of his mother. WE see him slaughter sandpeople, so when palpatine mentions it in Sith, there is a context for it. that isn't dependant on padme hearing about it.
i've never liked that the two of them discuss his slaughtering these sand people, if only because padme seems to have little to no reaction that makes sense. i mean, wouldn't just a hint of revulsion be appropriate? the guy lost his mother, and then responded to that by slaughtering wholesale an entire village of sentient beings. it's always pushed the boundaries of credibility for me, in an emotional and character sense.