The Palmer House - a portal?
Mrs Tremond / Chalfont gave Laura a picture that would look nice on her wall in FWWM that turned out to be a portal so there is confirmation that a portal exists in the Palmer house
100% agreed and even further, you will note that no one EVER removes that picture and...
Sarah is now inhabited by Judy/Experiment.
This could have happened at any point from then on, or she always was (bug in mouth during season 8)
The portal has been there since the picture from Tremond/Chalfont...Based on current and past owners of the house this is how things have transpired.
Is the picture there in ep 18?