\documentclass{memoir}

\usepackage{xpatch}

\renewcommand{\mempreaddchaptertotochook}{%
  \addtocontents{toc}{%
    \protect\settowidth{\protect\cftchapternumwidth}{\cftchapterfont \thechapter.\space}%
    \protect\setlength{\protect\cftsectionindent}{\protect\cftchapternumwidth}%
  }%
}

\makeatletter

\xpatchcmd{\@m@mschapter}% <cmd>
  {\@schapter}% <search>
  {\addtocontents{toc}{\protect\settowidth{\protect\cftsectionindent}{\protect\cftchapterfont \thechapter.\space}}%
     \@schapter}% <replace>
  {}{}% <success><failure>

\patchcmd{\M@sect}% <cmd>
  {\addcontentsline}% <search>
  {\addtocontents{toc}{%
     \protect\settowidth{\csname cft#1numwidth\endcsname}{\csname cft#1font\endcsname\csname the#1\endcsname.\space}%
     \protect\setlength{\csname cftsub#1indent\endcsname}{\protect\dimexpr\csname cft#1numwidth\endcsname+\csname cft#1indent\endcsname}%
   }%
   \addcontentsline}% <replace>
  {}{}% <success><failure>
\makeatother

%% delete boxes
\renewcommand\numberlinebox[2]{#2} % for sections
\renewcommand\chapternumberlinebox[2]{#2} % for chapters
%% set dot and space
\renewcommand\cftchapteraftersnum{.\space}
\renewcommand\cftsectionaftersnum{.\space}
\renewcommand\cftsubsectionaftersnum{.\space}
\renewcommand\cftsubsubsectionaftersnum{.\space}

%%enumeration levels in the ToC and text
\settocdepth{subsection}
\setsecnumdepth{subsubsection}
\setrmarg{2.55em plus1fil} %hyphenation is forbidden

\renewcommand{\cftchapterfont}{}

\begin{document}

\tableofcontents

\chapter*{Introduction}
\addcontentsline{toc}{chapter}{Introduction}
\section{First section}

\chapter{First chapter}
\section{First section  long long long long long long long long long long long long title}
\subsection{First subsection  long long long long long long long long long long long long title}
\setcounter{chapter}{11}

\chapter{Another chapter long long long long long long long long long long long long title}
\section{First section  long long long long long long long long long long long long title}
\subsection{First subsection  long long long long long long long long long long long long title}
\subsection*{Short conclusion with long long long long long long long long long long long long title}
\addcontentsline{toc}{subsection}{Short conclusion}

\end{document}