Problems shrinking margins with fancyhdr and memoir class

Question

Hullo all. I am typing/editing a doc using the memoir class and Bjournstrup fancy chapter headings. Whenever I try to adjust [shrink] the margins using the geometry package, 'odd' things happen: the bottom right corner of the greyed-in header which is 'behind' [for lack of a better word] the chapter number starts to disappear: bit-by-bit. Here is my attempt to reproduce a MWE:

The first is my ruleofprayer.tex file:

\documentclass[a4paper,12pt,twoside]{book}
\input{preamble}
\begin{document}
\include{chapter1}
\end{document}

Then, here is my preamble.tex file:

\usepackage{lmodern}
\usepackage{geometry}
\usepackage[dvipsnames]{xcolor}
\usepackage{amssymb}
\usepackage{framed}
\usepackage{indentfirst}
\usepackage{changes}
\usepackage[Bjornstrup]{fncychap}
\usepackage[pagestyles]{titlesec}
% % % % % % % % % % % % % % % % % % % % % % % % % % % % % %
\ChNumVar{\fontsize{76}{80}\usefont{OT1}{pzc}{m}{n}\selectfont}
\ChTitleVar{\raggedleft\Huge\sffamily\bfseries}

\titleformat{\section}[display]
{\normalfont\Huge\scshape\mdseries\centering}{}{16pt}{\Large}
\titlespacing*{\section}
{0pt}{20pt}{16pt}

\titleformat{\subsection}
{\normalfont\Large\bfseries\centering}{}{1em}{}
\titlespacing*{\subsection}
{0pt}{3.25ex plus 1.0ex minus 0.2ex}{1.5ex plus 0.2ex}

\titleformat{\subsubsection}
{\normalfont\large\scshape\mdseries\centering}{}{1em}{}
\titlespacing*{\subsubsection}
{0pt}{3.25ex plus 1ex minus .2ex}{1.5ex plus .2ex}

\titlespacing{\section}{0pt}{*2}{*2}
\titlespacing{\subsection}{0pt}{*2}{*2}
% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % 
\geometry{
  left=2.85cm,
  right=2.85cm,
  top=1.95cm,
  bottom=2.25cm
}
% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % 
\newpagestyle{main}{
\headrule
\sethead[\scshape\chaptertitle][][]{}{}{\slshape\chaptertitle}
\setfoot[][\thepage][]{}{\thepage}{}
}
\pagestyle{main}

And finally a brief portion of chapter1.tex:

\chapter{Saturday Evening Prayers}
Towards evening, we stand before the holy icons (having lit a candle and   maybe some incense), with reverence and fear of God; we gather our thoughts, make the Sign of the Cross, and say:

In the Name of the Father, and of the Son, and of the Holy Spirit. Amen.

\begin{framed}
From St. Thomas Sunday until Ascension, instead of \textbf{O God, cleanse me, a sinner \ldots we say:}
Christ is risen from the dead, trampling down death by death, and upon those in the tombs bestowing life.
\end{framed}

Afterwards, we collect our thoughts, and make seven prostrations (or metanoias), saying:

\section*{The Seven--Bow Beginning}

If I leave the geometry package alone (i.e. without any values assigned to it) the greyed-in background turns out as a nice rectangle as it should; only when I attempt to 'meddle' with the geometry and mess with the margins do I encounter the problem. Thanks!

I apologize for not properly splitting the files up before. I hope this clarifies the MWE.