I thought about the clue of the dollhouse that Marlene posted, have a look:
The doll at the top is dressed in the yellow top and jeans and look who is wearing the crown and look at Mona in the Season 5 finale, she wears the crown to the prom and she is wearing the same clothes as the doll.
How does Mona know everything about the house, she said ‘if you don’t make it back to your room in time, it will leave you without food and water for days and leave the siren on until you would rather die then live another second’. This doesn’t make sense because if she didn’t make it back in time and she was left without food and water she would be in a terrible state and she wasn’t in the house long enough to know that, and so how did she get back to her room and make the sirens go away.
Looking back at the picture again, look at the mona doll, her arms are out wide like she is saying welcome to my dollhouse. Why was Mona the only one to receive the gas mask, is this something to prepare herself to what’s coming. Also, A wouldn’t write ‘You’re my favorite’ to Alison or Mona, Emily was always the favorite to Alison and to A.
I think Mona is still playing the game but this time she is working with Charles. Charles could be using Mona to control the girls.