It was pretty rushed and so they didn't really have the diamonds take responsibility for the lives they destroyed for their empire. It was more focused on the family issues.

The diamonds weren't forgiven so much as had their records expunged so they could sing songs for the movie.

It was never realistic, but there was a distinct lack of average gems confronting the diamonds for their actions. The diamonds work to restore damage, but they can only do so much given the massive violence inflicted with thousands of years of imperialism on a galactic scale.