Just finished watching the finale, I did not see that reveal coming. At least one half of it at least, the other half was at least floated out as a theory early on in the series.
As a whole not sure how I feel, Darby and flashback Bill really carried the series. The present day guests at the retreat were all pretty thin to me, with none of them being that interesting. Some of the revelations around Andy/Lee towards the close of the series, really re-framed some early predictions I had.
Cool setting though.