vertical alignment for nested cells in multirow table

Question

This is what I came up with so far:

I'm trying to achieve the right column to be vertically centered over the parent rows like the cell containing "Puffer" so that the previous columns aren't affected. I don't really want a fixed length here, because I just want basically a list of items.

Unfortunately it's always aligned at the top and seems to be rendered just inside the first of the multirow cells which crashes the whole design.

I've already tried it with stack and itemize, but I seem to miss something more basic here.

Hopefully someone can help me out here because I can't find a proper or understandable solution anywhere.

\documentclass[12pt,a4paper]{article}
\usepackage[a4paper,lmargin={2.5cm},rmargin={2.5cm},tmargin={3cm},bmargin={2.5cm}]{geometry}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman, english]{babel} 

\usepackage{makecell}
\usepackage[table]{xcolor}
\usepackage{multirow}

\newcommand{\arr}{$\rightarrow$ }

\usepackage{array}
\usepackage{enumitem}
\usepackage{latexsym}
\usepackage{caption}


\begin{document}

\begin{table}[ht]
\centering
%\rowcolors{2}{gray!25}{white}
\begin{tabular}[c]{|c|c|c|}\cline{3-3}
\multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\\cline{3-3}
\hline
\multicolumn{1}{|c|}{
                    \multirow{4}{*}{12/2020}}&
                    1. Woche&
                    \multicolumn{1}{|c|}{\multirow{3}{*}{
                    
                    \makecell{\arr Test \\ \arr Test}
                    }}\\\cline{2-2}&
                    2. Woche&
                    \multicolumn{1}{|c|}{}\\\cline{2-3}&
                    3. Woche&
                    \multicolumn{1}{|c|}{\multirow{2}{*}{\arr Puffer}}\\\cline{2-2}&
                    4. Woche&
                    \multicolumn{1}{|c|}{}\\
                    \hline
\end{tabular}
\end{table}

\end{document}

Update:

I've added @Bernard's simplified version and added more text to it to see if it still gets aligned correctly. The code:

\begin{table}\setlength{\extrarowheight}{2pt}
    \begin{tabular}{|c|c|l|}\cline{3-3}
    \multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\\cline{3-3}
    \hline
                        \multirow{4}{*}{12/2020} & 
                        1. Woche &
                       \multirowcell{2}{\arr Prepare samples \\[2pt] \arr Implement Feedback}
                       \\\cline{2-2}&
                        2. Woche &
                        \\ \cline{2-3}&
                        3. Woche & \multirow{2}{*}{\arr Puffer} \\\cline{2-2}&
                        4. Woche & \\
                        \hline
    \end{tabular}
\end{table}

results in:

Answer 1

I propose this simpler code, with a left alignment of the right column (except for the column head):

\begin{table}\setlength{\extrarowheight}{2pt}
    \begin{tabular}{|c|c|l|}\cline{3-3}
    \multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\\cline{3-3}
    \hline
                        \multirow{4}{*}{12/2020} & %}&
                        1. Woche &
                       \multirowcell{2}{\arr Test \\[2pt] \arr Test}
                       \\\cline{2-2}&
                        2. Woche&
                        \\ \cline{2-3}&
                        3. Woche & \multirow{2}{*}{\arr Puffer} \\\cline{2-2}&
                        4. Woche & \\
                        \hline
    \end{tabular}
\end{table}

Edit: With longer items, the code works as expected:

\setlength{\extrarowheight}{2pt}
    \begin{tabular}{|c|c|l|}\cline{3-3}
    \multicolumn{2}{c|}{\multirow{1}{*}{}}&\multicolumn{1}{c|}{Inhalt}\\
    \hline
                        \multirow{4}{*}{12/2020} &
                        1. Woche &
                       \multirowcell{2}[0pt][l]{\arr Prepare samples\\ 
                       \arr Implement Feedback
                       }
                       \\\cline{2-2}&
                        2. Woche&
                        \\ \cline{2-3}&
                        3. Woche & \multirow{2}{*}{\arr Puffer} \\\cline{2-2}&
                        4. Woche & \\
                        \hline
    \end{tabular}

Answer 2

It seems that you want \arr Test\\ \arr Test to occupy three rows. If so, you need to let 1. Woche or 2. Woche to occupy two rows.

\documentclass{article}
\usepackage{multirow}
\usepackage{makecell}
\newcommand{\arr}{$\rightarrow$ }

\begin{document}
\begin{table}[ht]
\centering
\begin{tabular}{|c|c|c|}
\cline{3-3}
  \multicolumn{2}{c|}{} & \multicolumn{1}{c|}{Inhalt}\\
\hline
  \multirow{5}{*}{12/2020} & \multirow{2}{*}{1. Woche}
    & \multirowcell{3}{\arr Test\\ \arr Test} \\
  & &\\
\cline{2-2}
  & 2. Woche &\\
\cline{2-3}
  & 3. Woche & \multirow{2}{*}{\arr Puffer}\\
\cline{2-2}
  & 4. Woche &\\
\hline
\end{tabular}
\end{table}
\end{document}