Ich habe mal das Minimalbeispiel entsprechend ergänzt:
\documentclass[british]{scrbook}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear-icomp,sorting=nyt,dashed=false,maxcitenames=2,sortlocale=de_DE,url=true,isbn=true,doi=true,uniquename=init,giveninits]{biblatex}
\addbibresource{biblatex-examples.bib}
%%% Einfügen von authoryear Block in Literaturverzeichnis
\newcounter{mymaxcitenames}
\AtBeginDocument{%
\setcounter{mymaxcitenames}{\value{maxnames}}%
}
\renewbibmacro*{begentry}{%
\printtext[parens]{%
\begingroup
\defcounter{maxnames}{\value{mymaxcitenames}}%
\printnames{labelname}%
\setunit{\nameyeardelim}%
\usebibmacro{cite:labeldate+extradate}%
\endgroup
}%
\quad% or \addspace
\newline
}
%%%%
\DeclareDelimFormat{finalnamedelim}{\addspace\&\space}
\DeclareDelimFormat[bib,biblist]{multinamedelim}{\addsemicolon\space}
\DeclareDelimAlias*[bib,biblist]{finalnamedelim}{multinamedelim}
\begin{document}
\cite{sigfridsson,companion,aksin}
\printbibliography
\end{document}