Problems with table when using multicolumn and nested tabular

Question

I have two problems with a table as highlighted in red in the figure below.

1. First, on the left, the multirow cell containing the text "Number" is not aligned at the center of the cell. I believe this happens because I have used a tabular inside the cell containing the "Very long category name distributed on two rows"; I did this because I need this text to be on two rows and I do not know another way to do that.
2. The cell containing "Category" is obtained using multirow but as you can see on the right, there is no vertical bar. I have no idea why this happens.

Anyone knows how I can fix this? Here is my code with a MWE:

\documentclass[a4paper]{article}
\usepackage{multirow}

\begin{document}
\begin{table}[t]
\begin{center}

\begin{tabular}{|c|c|c|c|c|} 
 \hline
 \multirow{2}{*}{Number} & \multicolumn{4}{c}{Category} \\ \cline{2-5}  
 & \begin{tabular}{c} Very long category name \\ distributed on two rows \end{tabular}  & cat.\ A & cat.\ B & \begin{tabular}{c} Another very long name \\ on two rows \end{tabular}\\
 \hline
 abc & def & ghi & jkl & mno\\
 \hline
\end{tabular}

\end{center}
\end{table}
\end{document}  

Answer 1

You can also draw that table with {NiceTabular} of nicematrix with two advantages:

• Instead of \multirow, you have a command \Block; with that command (which may be used to merge cells vertically but also horizontally), you have to specify the number of rows (and the number of columns) whereas, with \multirow, you have to specify the number of physical lines (which requires sometimes tuning as in your case).

• In {NiceTabular}, you have a key hvlines which draws all the rules, excepted in the blocks.

\documentclass[a4paper]{article}
\usepackage{nicematrix}

\begin{document}
\begin{table}[t]
\begin{center}
\begin{NiceTabular}{ccccc}[hvlines]
    \Block{2-1}{Number} & \multicolumn{4}{c}{Category} \\ 
    & \begin{tabular}{c} Very long category name \\ distributed on two rows \end{tabular}  
    & cat.\ A & cat.\ B & \begin{tabular}{c} Another very long name \\ on two rows \end{tabular}\\
    abc & def & ghi & jkl & mno\\
\end{NiceTabular}
\end{center}
\end{table}
\end{document} 

However, you need several compilations since nicematrix uses PGF/Tikz nodes.

Answer 2

\documentclass[a4paper]{article}
\usepackage{multirow}

\begin{document}
    \begin{table}[t]
        \begin{center}
            
            \begin{tabular}{|c|c|c|c|c|} 
                \hline
                \multirow{3.5}{*}{Number} & \multicolumn{4}{c|}{Category} \\ \cline{2-5}  
                & \begin{tabular}{c} Very long category name \\ distributed on two rows \end{tabular}  & cat.\ A & cat.\ B & \begin{tabular}{c} Another very long name \\ on two rows \end{tabular}\\
                \hline
                abc & def & ghi & jkl & mno\\
                \hline
            \end{tabular}
            
        \end{center}
    \end{table}
\end{document}