Short header for a very long Section's title in beamer presentation R Markdown (Pandoc) - header

Question

I am working on a beamer_presentation which consists of several sections. The section titles are too long to fit into the CambridgeUS headline. I have already tried several options from related latex sources to use some short title for the section (say just "Chapter 1" instead of "Chapter 1 with a very-very-very-very-very long title" by \AtBeginSection{\title[Short title]{Long title}}) but yet can't figure out how to manage this in header-includes of R Markdown YAML. I set the section title through # in Markdown
---
title: Title for the whole presentation
subtitle: 'Presentation'
author:
- Author
institute: "Institution"
date: " `r format(Sys.Date(), '%B %d, %Y')`"
output:
beamer_presentation:
latex_engine: xelatex
toc: true
highlight: tango
theme: "CambridgeUS"
colortheme: "lily"
fonttheme: "serif"
slide_level: 3
keep_tex: true
header-includes:
- \setbeamertemplate{navigation symbols}{}
- \AtBeginDocument{\title[Presentation]{Title for whole presentation}}
- \renewcommand{\raggedright}{\leftskip=0pt \rightskip=0pt plus 0cm}
- \def\sectionname{Chapter}
- \AtBeginSubsection{}
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE, warning = FALSE, message = FALSE)
```
# Chapter 1 with a very-very-very-very-very long title
## Intro
### The first page

Answer 1

In beamer, this is trivial, you can use \section[short version]{long version}. But Markdown makes such an easy job excruciatingly difficult.
However you can trick markdown like this:
---
title: Title for the whole presentation
subtitle: 'Presentation'
author:
- Author
institute: "Institution"
date: " `r format(Sys.Date(), '%B %d, %Y')`"
output:
beamer_presentation:
latex_engine: xelatex
toc: false
highlight: tango
theme: "CambridgeUS"
colortheme: "lily"
fonttheme: "serif"
slide_level: 3
keep_tex: true
header-includes:
- \setbeamertemplate{navigation symbols}{}
- \AtBeginDocument{\title[Presentation]{Title for whole presentation}}
- \renewcommand{\raggedright}{\leftskip=0pt \rightskip=0pt plus 0cm}
- \def\sectionname{Chapter}
- \AtBeginSubsection{}
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE, warning = FALSE, message = FALSE)
```
```{=latex}
\tableofcontents[hideallsubsections]
\end{frame}
\section[short version for headline]{Chapter 1 with a very-very-very-very-very long title}
\subsection[short subsection for headline]{Intro}
\begin{frame}
\frametitle{The first page}
```
some text
(be careful where you place this command, otherwise you might end up with empty extra frames - that's why I had to switch off the automatic toc and insert it manually...)