This Post will likely get modified over the next few days
Takes Place Before Leaving Ellis House:
Erin was not aware of the tunnels under the Ellis House.
The Preservation Society served as a sort of guardian/gatekeeper for the house and part of Society6.
Membership into the S6 isn't apparently just "bought" but large contributions to the right individuals and organizations bought you greater access.
Matthew Platt - Head of the Preservation Society was falling out of favor with Ankroptik. She discovered this when going to see him about checking the grounds. She noticed he kept blowing his nose and carefully saving the tissues. She describes it as, "Ankroptik's presence leaving his body." (Basically he was blowing out maggot like creatures.)
She doesn't know why he was following out of favor but suspects it was because he had let the house grounds flood. Though it could have also just been for whatever other reason including maybe Ankroptik was just done with him.
Either way she saw it as a sign.
She happened to be there shortly after Victor Landry (One of the heads of S6) had met with him and traveled to Boston to catch up with him. Victor had also recently met with Mr. Owl, (who she knows little about beyond some info from Victor, and from having researched Gateway) but her closeness to the case got her able to see him. Victor invited her to come to the house for the attempted communion.
Her plan to knock the circle apart when Ankroptik arose was to appear as the only survivor and hence in Ankroptik's favor. She was sure she could get away and says, "The circle serves to protect everyone and confine him, but Ankroptik can only commune for short periods of time through the ritual. After fifteen minutes or so he'd return to his realm. He isn't ready to stay among us."
Erin will excuse herself at the end saying that she has to take care of some things quickly but will be touch very soon.
After Leaving the Ellis House
One thing that should sink in by now is that Erin seems underprepared and not as knowledgable as she might think she is. Equivalent wise it would like a devoted but "casual" Catholic suddenly trying to be a Bishop or Cardinal for the Catholic church. By "casual" I mean like a regular member of a congregation. She does strongly feel herself to be called on though.