I disagree with you. No, they couldn't have fit Adventures in the Sin Trade anywhere else. The reason why it was important was to help Xena's search of Gabrielle and how to bring her back. Obviously along the way she gets sidetracked with the Northern Amazons and gets a little caught up in her past. But, the other huge part of Adventures in the Sin Trade was her receiving a prophecy of her and Gabrielle's death thus leading her to know Gabrielle survived. Not to mention this random vision of them throughout season 4 of them getting crucified would have made no sense. I thought it was done well and fit fine in the series. Think about it if they did A Family Affair first, it would have screwed up the timeline of the episodes.
ah but then they could have done what they did with hercules, and make a half season or even just another season. now that would have made my day! ,more xena!