Formatting problems headings (TOC and document) - line spacing and spacing numbering

Question

I have to admit that I am a little bit desperate now, since I have been working the entire day on the following issues and was not able to come up with a proper solution.

1. I would like to have the chapters in roman numbering the rest should be in arabic. Unfortunately, the roman numbering is then too close to the heading in the ToC. (see III. and IV).
2. In addition, I cannot set the line spacing in the headings (if they have two or more lines) to single line spacing. (see I.3 in the ToC and in the document).
3. The subparagraph is unfortunately too far to the right and I don't know how to move it to the far left.

It would be great if anyone of you could help me, please.

MWE:

\documentclass[a4paper,
    bibliography=totoc,
    toc=listof,
    headings=small,
    listof=entryprefix,
    ]
    {scrbook}
%%% === PAKETE  === %%%
%% --- Allgemein --- %%
\usepackage[utf8]{inputenc}

\usepackage{textcomp}
\usepackage[]{setspace}

\usepackage[T1]{fontenc}

\usepackage[english,ngerman]{babel}

\usepackage{lmodern}

\usepackage{scrlayer-scrpage}

\usepackage{microtype}

\usepackage{ragged2e}
\usepackage[labelfont=bf,format=hang,justification=justified]{caption}
\usepackage{etoolbox}
\usepackage{pdflscape}
\usepackage{lscape}
\usepackage{ragged2e}

\usepackage{scrhack}
%% --- Zitieren / Fußnoten --- %%
\usepackage[babel,german=quotes,threshold=1]{csquotes}
\usepackage[language=autobib,
    backend=biber,                                                      % switch off if MWE is not working
    citetracker=true,
    autolang=other,

    style=authoryear-comp,

    maxcitenames=2,maxbibnames=99,

    uniquename=false,uniquelist=true,

    isbn=false,doi=false,

    dashed=false
    ]{biblatex}
% Fußnoten fortlaufend
    \usepackage{chngcntr}
    \counterwithout{footnote}{chapter}
    \usepackage{bigfoot}
% === Allgemein === %%
%\usepackage[onehalfspacing]{setspace}
    % Überschriften Tabellen / Abbildungen kleiner
    \captionsetup{font={small},justification=raggedright}
%% === Verzeichnisse === %%
% -- Inhaltsverzeichnis -- %
\setcounter{tocdepth}{5}


%% --- Überschriften --- %%

    \setcounter{secnumdepth}{5}

    \setkomafont{disposition}{\normalcolor\bfseries}

    \renewcommand{\partpagestyle}{empty}
    \renewcommand{\raggedsection}{\RaggedRight}
%% --- Abstände --- %%
    % Abstand zwischen Absätzen %
    \setlength{\parskip}{0.5\baselineskip} 
    \setlength{\parindent}{0pt}
%% --- Schusterjungen / Hurenkinder 
    \usepackage[defaultlines=3, all]{nowidow}
% Arabisch/Römisch Nummerierung
\renewcommand{\thechapter}{\Roman{chapter}}


% 5. Gliederungsebene
\newcommand{\myparagraph}[1]{\paragraph{#1}}
\RedeclareSectionCommands[beforeskip=.0\baselineskip,afterskip=.0\baselineskip]{chapter}
\RedeclareSectionCommands[beforeskip=-.5\baselineskip,afterskip=.1\baselineskip]{section,subsection,subsubsection,paragraph}
\RedeclareSectionCommands[beforeskip=-.5\baselineskip,afterskip=.1\baselineskip]{subparagraph}

%%% Größenanpassung Überschriften
\addtokomafont{chapter}{\fontsize{14}{17}\selectfont}
\addtokomafont{section}{\fontsize{12}{17}\selectfont}
\addtokomafont{subsection}{\fontsize{12}{17}\selectfont}
\addtokomafont{subsubsection}{\fontsize{12}{17}\selectfont}
\addtokomafont{paragraph}{\fontsize{12}{17}\selectfont}
\addtokomafont{subparagraph}{\fontsize{12}{17}\selectfont}






%*************************************************************************************
\begin{document}
%%% ----- VERZEICHNISSE ----- %%%
\pagenumbering{Roman}
\tableofcontents
\cleardoublepage 
\pagenumbering{arabic}
\onehalfspacing
%*************************************************************************************
\chapter{This is chapter number 1}
    \section{This is a new section}
    \section{This is an other one}
    \section{This is an other one, but the line spacing is very long and therefore looks strange. Any ideas how to fix this? (see page 1)}
        \subsection{Subsection}
         \subsubsection{Subsubsection}
            \paragraph{Paragraph}
            \subparagraph{Subparagraph}
\chapter{This is chapter number 2}
\chapter{This is chapter number 3}
\chapter{This is chapter number 4}
\chapter{This is chapter number 5}
enter image description here
%*************************************************************************************
\end{document}

Answer 1

First, use the usual font switches to set headings font size

\addtokomafont{chapter}{\Large}
\addtokomafont{section}{\large}
\addtokomafont{subsection}{\large}
\addtokomafont{subsubsection}{\large}
\addtokomafont{paragraph}{\large}
\addtokomafont{subparagraph}{\large}

in place of \fontsize (the second argument of \fontsize set the baseline skip, you can also use a smaller value). This would solve your issue #2.

For the table of content, as you have many levels, maybe the KOMA-Script flat style is more appropriate. You can set it with:

\KOMAoptions{toc=flat}

If you prefer keeping the default style, you can enlarge the numwidth for the chapter (issue #1):

\RedeclareSectionCommands[tocnumwidth=2em]{chapter}

and reduce the indent for the other levels (issue #3):

\RedeclareSectionCommands[tocindent=1em]{section}
\RedeclareSectionCommands[tocindent=2em]{subsection}
\RedeclareSectionCommands[tocindent=3em]{subsubsection}
\RedeclareSectionCommands[tocindent=4em]{paragraph}
\RedeclareSectionCommands[tocindent=5em]{subparagraph}