\include vs \usepackage

Question

I have some doubt about the correct approach to document modularity. Main main doubt is about the usage of custom packages in the creation of a complex structure document. In my first LaTeX work I made a custom package (.sty) with structure / commands definitions and other global settings. This allowed me to fix several compilation issues I had placing everything in a single file but I soon noticed some negative drawbacks about such approach:

• the custom .sty package referenced trough \usepackage directive is not found/recognized by TeXstudio editor (altough it is correctly compiled by LaTeX).

• compilation errors in custom package are 'mitigated' and anyway is often difficult to find the correct reference to the error.

• the templates / sample files I read learning LaTeX did not use such approach but often use \inlcude \input directives.

This's my document structure:

\documentclass[dvipsnames,a5paper,twoside,openright,italian,12pt]{memoir}

\usepackage{defines}

\author{The Author}
\date{2016}
\title{The Title}
\editor{The Editor}

\begin{document}

\frontmatter

\mytitle

\tableofcontents

\include{Preface}

\mainmatter

\include{Chapter01}

\include{Chapter02}

\end{document}

and this is my custom package (the name is defines.sty):

\usepackage[utf8]{inputenc} 
\usepackage[italian]{babel}
\usepackage[T1]{fontenc} 
\usepackage{verse}
\usepackage{background}
\usepackage[object=vectorian]{pgfornament}
\usepackage{svg}
\usepackage{wallpaper}
\usepackage[osf]{libertine}

\usetikzlibrary{backgrounds, calc}

\newcommand{\editor}[1]{
    \def\@editor{#1}
}

\newcommand{\theeditor}{
    \@editor
}

\newcommand{\eachpageornament}{
    \begin{tikzpicture}[remember picture, overlay, color=LimeGreen]
        \transparent{0.75}{     
            \node[anchor=north west](CNW) at (current page.north west){
                \pgfornament[width=2cm]{61}};
            \node[anchor=north east](CNE) at (current page.north east){
                \pgfornament[width=2cm,symmetry=v]{61}};
            \node[anchor=south west](CSW) at (current page.south west){
                \pgfornament[width=2cm,symmetry=h]{61}};
            \node[anchor=south east](CSE) at (current page.south east){
                \pgfornament[width=2cm,symmetry=c]{61}};
            \pgfornamenthline{CNW}{CNE}{north}{87}
            \pgfornamenthline{CSW}{CSE}{south}{87}
            \pgfornamentvline{CNW}{CSW}{west}{87}
            \pgfornamentvline{CNE}{CSE}{east}{87}
        }
    \end{tikzpicture}
}

\newcommand{\plogo}{
    \includegraphics[width = 30mm]{logo.png}
}

\newcommand{\psignature}{
    \begin{flushright}
        \def\svgwidth{40mm}
        \input{signature.pdf_tex}
    \end{flushright}
}

\newcommand{\mytitle}{
    \thispagestyle{empty}
    \ThisCenterWallPaper{1.1}{sfondo}

    \begin{tikzpicture}[remember picture, overlay]
        {\transparent{0.95}\node [rectangle, rounded corners, fill=LimeGreen, anchor=south west, minimum width=6cm, minimum height=8cm] (box) at (-0.5,-10) (box){};}
        \node[anchor=west, xshift=-2.0cm, yshift=-1cm, text width=4cm] at (box.north){\large \textit{\theeditor}}; 
        \node[anchor=west, xshift=-2.0cm, yshift=-3.5cm, text width=4cm] at (box.north){\huge \thetitle};
        \node[anchor=west, xshift=-2.0cm, yshift=-6cm, text width=4cm] at (box.north){\large \theauthor};
    \end{tikzpicture}

    \newpage
}

\backgroundsetup{
    contents={
        \if@mainmatter 
            \eachpageornament \thepage
        \else
        \ifnum\value {page}=1 
        \else 
            \eachpageornament \thepage
        \fi
        \fi
    },
    position=current page.north east,
    angle=0,
    scale=1,
    opacity=1
}

\renewcommand{\poemtoc}{section}
\renewcommand{\poemtitlefont}{\normalfont\large\itshape\centering} 

\pagestyle{plain}

\setcounter{tocdepth}{2}

If I rename the package in .tex and include it (trough \include or \input) I got several errors.

I have another doubt about the use of memoir class instead book ... If I use the second instead the first one I got a number of errors regarding references to commands defined in the custom package.