Like this?

\documentclass{beamer}
\mode<presentation>
{
\usetheme{default} % or try Darmstadt, Madrid, Warsaw, ...
\usecolortheme{rose} % or try albatross, beaver, crane, ...
\usefonttheme{default} % or try serif, structurebold, ...
\setbeamertemplate{navigation symbols}{}
\setbeamertemplate{caption}[numbered]
}
\usepackage{collcell} %pdflatex.exe hangs without this one
\usepackage[customcolors,beamer]{hf-tikz} %for beautiful inline highlighted math. Thanks Claudio!
\AtBeginSection[]{
\begin{frame}
\vfill
\centering
\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
\usebeamerfont{title}\insertsectionhead\par%
\end{beamercolorbox}
\vfill
\end{frame}
}
\newcommand\fiteq[1]{%
\sbox{\mybox}{$\displaystyle#1$}%
\ifdim\wd\mybox>.85\textwidth\resizebox{.85\textwidth}{!}{\usebox{\mybox}}%
\else\usebox{\mybox}\fi%
}
\title{My title: \\
and here is the subtitle}
\date{today's date}
\author{
A1\inst{1}, \parbox[t]{22mm}{\centering A2\inst{2},
\includegraphics[width=15mm]{example-image-duck}}
\parbox[t]{22mm}{\centering A3\inst{3},
\includegraphics[width=15mm]{example-image-duck}}
\parbox[t]{22mm}{\centering A4\inst{2},
\includegraphics[width=15mm]{example-image-duck}}
\parbox[t]{22mm}{\centering A5\inst{1}
\includegraphics[width=15mm]{example-image-duck}}
}
\institute{\inst{1} fancy university 1 \and \inst{2} not-so-fancy university 2 \and \inst{3} dream-place-to-work-at}%\setbeamercolor{normal text}{fg=myblue}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
Adendum:*
As you required in comment below:

\documentclass{beamer}
\usepackage[export]{adjustbox} %pdflatex.exe hangs without this one
\title{My title: \\
and here is the subtitle}
\date{\today}
\author{\setkeys{Gin}{width=15mm}
A1\inst{1} \\[2ex]
\parbox{.45\linewidth}{Short Name\inst{2}\hfill
\includegraphics[valign=c]{example-image-duck}}\hfill
\parbox{.45\linewidth}{A. B. Name3\inst{3}\hfill
\includegraphics[valign=c]{example-image-duck}} \\[2ex]
\parbox{.45\linewidth}{Forename Lastname\inst{2}\hfill
\includegraphics[valign=c]{example-image-duck}}\hfill
\parbox{.45\linewidth}{My name is Long\inst{1}\hfill
\includegraphics[valign=c]{example-image-duck}}
}
\institute{\inst{1} fancy university 1 \and \inst{2} not-so-fancy university 2 \and \inst{3} dream-place-to-work-at}%\setbeamercolor{normal text}{fg=myblue}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}
(in above MWE is removed all definitions and packages irrelevant to your problem)
One more addendum:
\documentclass{beamer}
\title{My title}
\subtitle{here is the subtitle}
\date{\today}
\author{
A1\inst{1}, Short Name\inst{2}, A. B. Name3\inst{3}, Forename Lastname\inst{2} and My name is Long\inst{1}
}
\institute{\inst{1} fancy university 1, \inst{2} not-so-fancy university 2, \inst{3} dream-place-to-work-at}%\setbeamercolor{normal text}{fg=myblue}
\begin{document}
\begin{frame}
\titlepage
\vfill
\centering
\setkeys{Gin}{width=15mm}
\includegraphics{example-image-duck}\hfil
\includegraphics{example-image-duck}\hfil
\includegraphics{example-image-duck}\hfil
\includegraphics{example-image-duck}
\end{frame}
\end{document}

Hopefully is now enough examples how can you format title frame of your presentation.