I totally agree that the new area needs a bonus to encourage players to fatten as many girls as possible to the final weight stages.
What I'm thinking is tying it into the potential new advancement/promotion mechanic. You'll get a small bonus for each milestone & then a big bonus once you reach the final scene. There would be a large penalty for moving a girl back to the care home to prevent players from obtaining the milestone bonuses, then inducing weight loss to earn the care home's affection bonus.
Your best bet to remove the duplicate wife might be to make her mobile again, then remove her from the game via the delete button in the phone (make sure you do a manual save first to be safe).
My original design behind the milestones was to allow players to continue to be rewarded for weight gain for as long as they wish. I added that final scene to mark the end of the dialogue & description changes. I'll add some text to explain this.