You may actually need maxed-out friendship/rivalry to get that last scene. According to the guide (which I don't have, but someone posted on BSN):

2: Complete remaining companion quest and dialogues, give him both gifts, reach 100% friendship or rivalry, then complete the Questioning Beliefs final-stage relationship conversation before the start of The Last Straw.

I honestly think that's your issue, not the slaver girl. Might not be too difficult to fix - can you skip back a few quests and choose something to piss him off? A pain, I know, but definitely not as bad as having to go back to Act 2!