Bibliography with biblatex: how to achieve bold labels using the authoryear-style?

Your example wasn't quite minimal, so I removed some redefinitions not related to the actual problem (achieving bold labels). I suggest to simply wrap the author bibmacro (which also prints the year in authoryear styles) into a \mkbibbold -- I use the xpatch package to do so. (In addition, one has to make the colon after the year bold by redefining \labelnamepunct.)

EDIT: For works without author, the bbx:editor bibmacro must be changed accordingly.

\documentclass{scrreprt}

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[babel,german=quotes]{csquotes} 

\usepackage[style=authoryear]{biblatex}
\ExecuteBibliographyOptions{firstinits=true,maxnames=2}

\setlength{\bibitemsep}{18pt}
\setlength{\bibhang}{0.2cm}

\DeclareNameAlias{sortname}{last-first}

\AtBeginBibliography{%
  \renewcommand*{\multinamedelim}{\addsemicolon\space}%
  \renewcommand*{\finalnamedelim}{\addsemicolon\space}%
}

\DefineBibliographyStrings{ngerman}{%
  andothers = {et\addspace al\adddotspace},%
  andmore = {et\addspace al\adddotspace},%
}

\usepackage{xpatch}
\xpretobibmacro{author}{\mkbibbold\bgroup}{}{}
\xapptobibmacro{author}{\egroup}{}{}
\xpretobibmacro{bbx:editor}{\mkbibbold\bgroup}{}{}
\xapptobibmacro{bbx:editor}{\egroup}{}{}

\renewcommand*{\labelnamepunct}{\mkbibbold{\addcolon\space}}

\usepackage{filecontents} 

\begin{filecontents*}{\jobname.bib} 
@article{nature, 
   author      =   {Rosa Rademakers and Manuela Neumann and Ian R. Mackenzie}, 
   title      =   {Advances in understanding the molecular basis of frontotemporal dementia - elongated title}, 
   journal      =   {Nature Reviews Neurology}, 
   volume      =   {8}, 
   year      =   {2012}, 
   pages      =   {423-434}, 
   doi         =   {10.1038/nrneurol.2012.117},
} 
@article{fuente, 
   author      =   {D. de la Fuente and J.G. Castaño and M. Morcillo}, 
   title      =   {Long-term atmospheric corrosion of zinc}, 
   journal      =   {Corrosion Science}, 
   volume      =   {49}, 
   year      =   {2007}, 
   pages      =   {1420–1436},
}
 @incollection{Rocca2007a,
   author = {Rocca, Emmanuel and Mirambet, Fran\c{c}ois},
   booktitle = {Corrosion of Metallic Heritage Artefacts: Investigation, Conservation and Prediction of Long Term Behaviour},
   chapter = {18},
   editor = {Philippe Dillmann and Pedro Piccardo and Gerard Beranger},
   pages = {308-334},
   title = {Corrosion inhibitors for metallic artefacts: temporary protection},
   year = {2007},
}
@misc{A01,
  editor = {Author, A.},
  year = {2001},
  title = {Alpha},
}
\end{filecontents*} 

\addbibresource{\jobname.bib} 

\nocite{*}

\begin{document}

\printbibliography

\end{document}