How to return to “the mad God’s bargain” mission

The song is a rather patriotic tune about the formation of the Ebonheart Pact, so firstly, you're gonna need to be in one of their cities. It's just not the sort of thing a Covenant or Dominion bard is ever gonna sing.

Beyond that, it should be possible to hear it in basically every tavern where someone sings; all the bard songs in the game were recorded by numerous voice actors, so every bard voice can sing every possible song. You may, however, need to wait a short while for that particular song to come up.

The first place where I can outright guarantee that the song plays, is in Davon's Watch, the city you initially spawn in upon leaving the tutorial as a Pact character. The tavern there, known as The Watch House, has a bard singing in the kitchen area on the right, and she does occasionally sing Three Hearts As One. It's right near the temple/mages guild, and across the street from a bank.

Unfortunately, I don't know how they handled translating the songs; the German version may be completely different.

The only difference between saving Valaste or not saving her is the two skill points. You do not get the points if you save her, and you are not rewarded with anything else for saving her.