COMMENTS ON LATEST EDIT:
In response to Mr. Clark's stipulation that he didn't like the \vskips, those have been successfully removed. Further, all spacings are issued through the titlesec package and are now specified in terms of \baselineskip, which should provide more flexigility for fontsize changes. Significant changes in this edit were several. First, changing the titlesec format from "display" to "hang" accomplished a great deal. However, the issue of \lineskiplimit proved to be decisive. The \boxedsection command created by Jeff was of a size not close to an even multiple of \baselineskip and so LaTeX kept adding a line to correct a perceived problem. Setting the line-skip-limit to a more negative value resolved the issue. And it allowed me to use just the titlesec parameters to achieve the result, without any vspaces.
Nonetheless, there remained the issue of glue, to which I alluded in one of the comments. I was largely able to remedy it by setting \parskip to 0 and setting \textfloatsep so that it couldn't grow (I'm not wholly sure I'm using that command properly). However, a super-zoom on the flushed section headings can still occasionally perceive some glue-gap.
Maybe there's another glue parameter that can help.
COMMENTS ON AN EARLIER EDIT:
In response to the comment that there probably needs to be no separate commands used for the flushed sub- and subsub-section headings, I have remedied that problem, with one proviso. The method works by counting how many \pars have been issued after the higher-level section invocation. It starts by assuming you have a flush-heading sub*section. Then it starts counting \pars. Once it reaches a critical number, it concludes that you have added text to the section , and changes the \sub*section heading for the next lower heading level. Therefore, the user must be consistent in how they add text to a section. Currently, the triggers are set up for the following structure
\section{name}
<blank line>
text
<blank line>
\subsection{name}
<blank line>
text
<blank line>
\subsubsection{name}
or for flush headings
\section{name}
<blank line>
\subsection{name}
<blank line>
\subsubsection{name}
Thus, there remains the requirement for the user to type his section headings consistently. And because of the feature of texexchange to squeeze blank lines out of listings, the needed blank lines in the body do not show up in the below listing.
\documentclass{scrbook}
\usepackage[letterpaper,asymmetric,left=108pt,right=54pt,top=41pt,bottom=41pt,headsep=13pt,headheight=14pt,footskip=23pt,marginparwidth=72pt]{geometry}
\usepackage[compact,explicit,calcwidth]{titlesec}
\usepackage{tikz,lipsum}
\usetikzlibrary{calc}
\usepackage{ifthen}
%
\usepackage{setspace}\onehalfspacing
%
\setkomafont{sectioning}{\normalfont}
%
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
%
\newlength\titlemarginwidth
\setlength\titlemarginwidth{54pt}
%
\newcommand\boxedsection[6]{{%
\usekomafont{sectioning}\usekomafont{#2}%
\begin{tikzpicture}[inner sep=0, inner ysep=0.7ex, node distance=0]%
\node[anchor=base west,rectangle,text
width=\titlemarginwidth-\columnsep-1pt,color=#3] at (0,0) (numnode)
{\vphantom{yZ}\hspace{1ex}#6};%
\node[anchor=base west,rectangle,text width=\textwidth-.5pt] at
($(numnode.base east)+(\columnsep,0)$) (textnode) {\vphantom{yZ}#1};%
\begin{pgfonlayer}{background}%
\fill[color=#4] (0,0|-numnode.north) rectangle (numnode.south
east|-textnode.south east);%
\draw[#5,semithick] (0,0|-numnode.north) -- (textnode.north
east);%
\draw[#5,semithick] (0,0|-textnode.south west) -- (textnode.south
east);%
\end{pgfonlayer}%
\end{tikzpicture}%
}}%
\newcommand\flushboxedsection[6]{{%
\usekomafont{sectioning}\usekomafont{#2}%
\begin{tikzpicture}[inner sep=0, inner ysep=0.7ex, node distance=0]%
\node[anchor=base west,rectangle,text
width=\titlemarginwidth-\columnsep-1pt,color=#3] at (0,0) (numnode)
{\vphantom{yZ}\hspace{1ex}#6};%
\node[anchor=base west,rectangle,text width=\textwidth-.5pt] at
($(numnode.base east)+(\columnsep,0)$) (textnode) {\vphantom{yZ}#1};%
\begin{pgfonlayer}{background}%
\fill[color=#4] (0,0|-numnode.north) rectangle (numnode.south
east|-textnode.south east);%
\draw[#5,semithick] (0,0|-textnode.south west) -- (textnode.south
east);%
\end{pgfonlayer}%
\end{tikzpicture}%
}}%
%
\titlespacing*{\chapter}{-0.7\titlemarginwidth}{50pt}{40pt}
\titleformat{\chapter}[hang]%
{\usekomafont{sectioning}\usekomafont{chapter}}%
{\emph{\chaptertitlename\ \thechapter}}%
{0em}%
{\emph{#1}}%
\titlespacing*{\section}{-\titlemarginwidth}{.6\baselineskip}{*0}%
\titleformat{\section}[hang]%
{\setstretch{1.6}}%
{}{0pt}{\boxedsection{#1}{section}{white}{black}{black}{\thesection}%
}
\def\tsubsectionbox{%
\titlespacing*{\subsection}{-\titlemarginwidth}{.8\baselineskip}{*0}%
\titleformat{\subsection}[hang]%
{}%
{}{0pt}{%
\boxedsection{##1}{subsection}{black}{gray!30}{black}{\thesubsection}}%
}
%
\def\fsubsectionbox{%
\titlespacing*{\subsection}{-\titlemarginwidth}{*0}{.2\baselineskip}%
\titleformat{\subsection}[hang]%
{\setstretch{1.38}}%
{}{0pt}{%
\flushboxedsection{##1}{subsection}{black}{gray!30}{black}{\thesubsection}}%
}
%
\newcounter{subparcounter}
\let\savesection\section
\def\section{\fsubsectionbox\setcounter{subparcounter}{0}\savesection}
%
\def\tsubsubsectionbox{%
\titlespacing*{\subsubsection}{-\titlemarginwidth}{.8\baselineskip}{*0}%
\titleformat{\subsubsection}[hang]%
{}%
{}{0pt}{%
\boxedsection{##1}{subsubsection}{black}{white}{black}{\thesubsubsection}}%
}
%
\def\fsubsubsectionbox{%
\titlespacing*{\subsubsection}{-\titlemarginwidth}{*0}{0\baselineskip}%
\titleformat{\subsubsection}[hang]%
{\setstretch{1.1}}%
{}{0pt}{%
\flushboxedsection{##1}{subsubsection}{black}{white}{black}{\thesubsubsection}}%
}
%
\newcounter{subsubparcounter}
\let\savesubsection\subsection
\def\subsection{\fsubsubsectionbox\setcounter{subsubparcounter}{0}%
\savesubsection}
%
\let\savepar\par
\def\par{%
\savepar%
\addtocounter{subsubparcounter}{1}%
\addtocounter{subparcounter}{1}%
\ifthenelse{5 = \value{subsubparcounter}}%
{\tsubsubsectionbox}%
{}%
\ifthenelse{5 = \value{subparcounter}}%
{\tsubsectionbox}%
{}%
}%
%
% SEE: http://www.tug.org/TUGboat/tb28-1/tb88bazargan.pdf
\lineskiplimit = -12pt % <--MAY NEED TO BE MADE MORE NEGATIVE!
\parskip 0ex
\setlength{\textfloatsep}{20pt plus 0pt minus 4pt}
%
\begin{document}
\chapter{Test}
\section{Section}
\lipsum[2]
text
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
Text
\section{Section}
Text
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\lipsum[2]
\section{Section}
\lipsum[2]
text
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
Text
\section{Section}
Text
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\lipsum[2]
\section{Section}
\lipsum[2]
text
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
Text
\section{Section}
\subsection{Subsection}
Text
\subsubsection{Subsubsection}
Text
\lipsum[2]
\end{document}
