People who rewound it to check said higher up in the thread that the severed head was definitely the leprechaun's. The way the Monster talked about the girl in the bar reminding him of his sister, I think he's keeping her for his sister to possess. Remember, Penny said the girl in the flashback *wasn't* a sacrifice for Enyalius.
I think Margo was referring to the demon herself doing laundry if requested, just as she gathered the black grains for Margo. They possess men to render them helpless, not to make them do chores. She said, "From now on if a demon possesses a man or a woman, you can just tell them to leave. Or stay and finish them off. Or help with the laundry." Meaning "Tell them (demon) to leave. Or tell them (demon) to stay and finish them (possessed) off. Or tell them (demon) to help with the laundry", not "Or tell them (demon) to make them (possessed) help with the laundry."
You mean "Narnia of the Calormenes reference ala The Horse and His Boy", right? I assume you mean that and not "Narnia of the Telmarines reference ala Prince Caspian", since you mentioned Aravis, but I'm not 100% sure since both are imperialist and the Telmarines actually conquered Narnia, so there was a Telmarine-era Narnia, hence "Narnia of the Telmarines".
Was Sera Gamble involved in Whedon shows? I don't see any mentioned in her history on Wikipedia or IMDB, or by Googling. Could you be confusing her with Jane Espenson (who was involved in Buffy, Angel, Firefly and Dollhouse)?