Eliminate \hbox badness in multiple verse-like environment

Question

This is a question dealing with underfull \hbox badness when newlines are entered using \\.

\documentclass[12pt]{memoir}
\usepackage{lipsum}
\usepackage{polyglossia} % the multilingual support package

\setromanfont[Ligatures=TeX]{Linux Libertine O}
\def\mverse{\ifhmode \par \fi
   \goodbreak \addvspace{3ex} \refstepcounter{section}
   \centerline{\textbf{Verse \arabic{section}}}\medskip}

\newenvironment{customtext}%
  {\leftskip=1em \rightskip=1em }%
  {\ifhmode \par \fi}
\newcommand\mstyle[1]{\begin{customtext}\setlength{\parindent}{0pt}{\large
    \textbf{#1}} \\
    {\sffamily #1}\end{customtext}\medskip}

\begin{document}

\mverse
\mstyle{This is a custom verse}
\lipsum

The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.

\mverse
\mstyle{
This is verse 2 line 1\\
The is verse 2 line that is a long line in for one type of fonts and can be accommodated in a single line %adding a \\ at the end of line will cause hbox badness
}

The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.
The old fox jumps over the lazy dog.

\end{document}

the mstyle macro can take more one than one line of verse separated by \\ and then correspondingly display in multiple languages. That's the intent. However this produces widespread underfull \hbox.

How can hbox badness be avoided in such cases? The discussion: How to properly code a TeX file, or at least avoid `badness 10000` offered some insight that a multline environment may prevent it. I am unable to wrap them in a multline begin/end block. Are there alternatives?