First, Carmen requires a box of chocolates that you can purchase from the Gym after you:
- Trick Eve to start gaining weight by leaving a chocolate bar in her office.
- Meet Bonnie in the city and are given a specialty box of chocolates
- Give that box of chocolates to Eve
- Wait for Eve to gain weight
- Purchase the 'normal' box of chocolates for Carmen's quest.
Second, The final maid is most likely on the train and randomly may show up there.
Finally, no, you must choose between one of the three. There is no harem house ending.