Wait… can you actually progress without being a member of one of the guilds…?
As for doing it in front of your slave requiring a boring explaination; instead of having a region in your bedroom, have a region in the rooms the slave can occupy, (aside from cutscenes where you can't use the menu, I've only seen them in the bedroom) and dissalow the milk machine use in that region. Also do this for other characters that may appear in the house. That way, if there isn't an NPC in the room, you'be able to use the milk machine.
The reason I ask about the merchant slave route is because I was expecting Zanti workers to show up, seeing that I wouldn't be sending them milk or money any more, and would have their glowy tracker tattoo… the whole thing would conflict with itself…
wouldn't it?