I think I have met all your requires here:

I have created a \newcommand called \annot (for your annotations, because that text looks like a screenplay or some theatrical text). You just need to put \annot{the text you want to put in the annotation} and it will generate the square brackets, the centering inside, and centering outside if the length of any line in the annotation is greater than 4cm. Maximum width inside the annotations is 13cm, then it jumps the line (if you haven't do it manually).
\annot can split at the end of the page, "cutting" the square brackets in two pieces. But this system is not perfect. It works as follows:
Multiline with 1 or more lines of 13cm splits well:

Multiline with all the lines less than 13 cm can't preserve hbox so the brackets are not adjusted to the text, but It can splits well too:

If you have a short annotation (<= 4cm) followed by normal text, like ("Zum Neger". Ab!). You just need to put the text in a second pair of curly brackets: \annot{Zum Neger}{Ab!}
Code:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[german,activeacute]{babel}
\usepackage{varwidth}
\usepackage[many]{tcolorbox}
\tcbset{
bracketstyle/.style={fonttitle=\bfseries, breakable, freelance, colback=white, colframe=white,
toprule=0pt, bottomrule=0pt, boxrule=0pt, halign=center}
}
\tcbset{
mstyle/.style={
bracketstyle,
text width=13cm,
overlay unbroken and first={
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=18pt]frame.north west) --
([yshift=-5pt,xshift=14pt]frame.north west) --
([yshift=5pt,xshift=14pt]frame.south west);
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=-18pt]frame.north east) --
([yshift=-5pt,xshift=-14pt]frame.north east) --
([yshift=5pt,xshift=-14pt]frame.south east);
},
overlay unbroken app={
\draw[black!65!white,line width=0.7pt,line cap=rect]
([yshift=5pt,xshift=18pt]frame.south west) --
([yshift=5pt,xshift=14pt]frame.south west);
\draw[black!65!white,line width=0.7pt,line cap=rect]
([yshift=5pt,xshift=-18pt]frame.south east) --
([yshift=5pt,xshift=-14pt]frame.south east);
},
overlay middle and last={
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=14pt]frame.north west) --
([yshift=5pt,xshift=14pt]frame.south west);
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=-14pt]frame.north east) --
([yshift=5pt,xshift=-14pt]frame.south east);
},
overlay last app={
\draw[black!65!white,line width=0.7pt,line cap=rect]
([yshift=5pt,xshift=14pt]frame.south west) --
([yshift=5pt,xshift=18pt]frame.south west);
\draw[black!65!white,line width=0.7pt,line cap=rect]
([yshift=5pt,xshift=-18pt]frame.south east) --
([yshift=5pt,xshift=-14pt]frame.south east);
}
}
}
\newtcolorbox{mannot}{
mstyle
}
\newtcolorbox{lmannot}{
mstyle, hbox, left=14pt, right=14pt
}
\newtcolorbox{sannot}[1]{
bracketstyle,
hbox,
overlay unbroken={
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=16pt]frame.north west) --
([yshift=-5pt,xshift=12pt]frame.north west) --
([yshift=5pt,xshift=12pt]frame.south west) --
([yshift=5pt,xshift=16pt]frame.south west);
\draw[black!65!white,line width=0.7pt]
([yshift=-5pt,xshift=-16pt]frame.north east) --
([yshift=-5pt,xshift=-12pt]frame.north east) --
([yshift=5pt,xshift=-12pt]frame.south east) node [yshift=0.315cm,black,right] {#1} --
([yshift=5pt,xshift=-16pt]frame.south east) ;
}
}
\newcommand{\annot}[2]{
\sbox0{#1}
\ifdim\wd0>4cm\relax
\ifdim\wd0<13cm\relax
\begingroup\centering\begin{lmannot}{#2}\begin{varwidth}{\textwidth}\centering \emph{#1} \end{varwidth} \end{lmannot} \endgroup
\else
\begingroup\centering\begin{mannot} \emph{#1} \end{mannot}\endgroup\fi
\else
\begin{sannot}{#2} \emph{#1} \end{sannot}\fi
}
\usepackage{vmargin}
\setmargins{3cm} % Left margin
{1.3cm} % Top margin
{15.6cm} % Text width
{23.42cm} % Text height
{10pt} % Headers height
{1cm} % Space between headers and text
{0pt} % Footnote height
{2cm} % Space between footnote and text
\begin{document}
\annot{Schnell ab.\ Kurze Pause. Leontine tritt von der Tür zurück, legt ab. Der Kellner öffnet die Tür von Garilan. Garilan tritt auf, verbeugt sich kurz vor Leontine. Der Neger, mit Garilans riesigem, zettelverklebtem Koffer beladen, folgt Garilan und dem Kellner.}
\noindent
KELLNER: Es sind unsere besten Räume, mein Herr -\
GARILAN, \emph{zu Leontine}: Die Karawane steht übermorgen bereit
\annot{Zum Neger.}{Ab!} \annot{Neger setzt den Koffer ab.}
\noindent
KELLNER: Zu den Schlafzimmern, zum Bad, mein Herr -\
GARILAN, \emph{zum Neger}: Wie viel?\
NEGER, \emph{zeigt >>sechs<<}: Sitta.\
GARILAN, \emph{bedeutet}: Zu viel!\
NEGER, \emph{zeigt >>fünf<<}: Chámsa.\
GARILAN, \emph{bedeutet}: Zu viel!\\
\hrule\bigskip\bigskip
\noindent
GARILAN: Sacre nom de Dieu! Porco di diavolo! Allah il Allah! God save the King!\
LEONTINE: Du wirst dich beschweren, Liebling!
\annot{Sie trägt Koffereinlangen ins Schlafzimmer. \ Garilan öffnet.}
\noindent
GARILAN: Was wollen Sie denn?
\end{document}