What is a robust user-defined way to define cross-references and adjunct values that work inside and outside environments?

Question

This question is a follow-up on these two

• Why does \zifrefundefined fail on tcolorbox reference?
• \@nameuse fails on definition within tcolorbox

What I want to achieve:

• I want to have an environment, e.g. tcolorbox, in which I have
• a way to define some kind of label that works with \vref or similar apporaches for having cross-references working back and forth, i.e. order must not matter
• a way to define additional variables connected to the label, something like <label-name>@definition
• I need to be able to check whether a certain label variable is available
• The approach must be stable against position in text, especially work inside or outside of environments

What I have learned so far:

zref

• zref works outside of environments like a charm. You can use \zref@newprop, \zref@setcurrent, \zref@extractdefault and \zref@labelbyprops to achieve astonishing results. \zifrefundefined helps top make your compilation run through even when something is missing.
• zref does not work within environments. I would appreciate something like zref but working within environments.

etoolbox

• etoolbox (which has only a documentation that works for very experienced [La]TeX users) is totally robust with environments. Using \csgdef and \csuse brings you very far.
• I am missing the complement feature of \zifrefundefined, but that is probably me not understanding the docs
• etoolbox has a caveat: order matters. Things have to be defined before first use.

Summary

I need the robustness for environments of etoolbox combined with the robustness of zref with regard to order of occurrence. Here's my testbed:

\documentclass[
  a4paper,
  11pt,
  parskip=half
]{scrbook}

\usepackage[nospace]{varioref}
\usepackage[pdfencoding=unicode, psdextra]{hyperref}
\usepackage{cleveref}
\usepackage[user]{zref}
\usepackage{tcolorbox}
\usepackage{etoolbox}

\tcbuselibrary{breakable}

\DeclareNewTOC[
  category=float,
  float,% declares floating environment eq
  floatpos=ht,
  nonfloat,% declares non-floating environment eq-
  listname={Some Items},
  name=TM,
  tocentrystyle=tocline,
  tocentrylevel:=table,
  tocentryindent:=table,
  tocentrynumwidth:=table,
  type=MySpecialEnvFloat,
]{ltm}

\newtcolorbox[
    auto counter,
    crefname={TM}{TMs}]{MyBox}[2][]{
    title={TM\,\thetcbcounter: #2},
    #1}
  
\newcounter{MySpecialEnvCounter}

    
\begin{document}

\makeatletter
\zref@newprop{Definition}{}

\newcommand{\MySpecialEnv}[4]{%
    \refstepcounter{MySpecialEnvCounter}%

    % etoolbox
    \csgdef{#1@Definition}{#2}

    % 
    \zlabel{#1}
    \zref@setcurrent{Definition}{#2}

    \begin{MyBox}[label={#1}]{#2}
        \addcontentsline{ltm}{section}{TM\,\theMySpecialEnvCounter: #2}

        Some text
        \tcblower
        #4
    \end{MyBox}
}%  
\makeatother

\listofMySpecialEnvFloats

\MySpecialEnv{test-id}{Something that looks like a summary}{-}{
    sdjoajhsja sijaoihs siahskah

    Nameuse: \@nameuse{test-id-2@Definition}

    Csname: \emph{\csname test-id-2@Definition\endcsname}

    % Order matters for etoolbox, so this one will fail
    Etoolbox: \csuse{test-id-2@Definition}
}

\MySpecialEnv{test-id-2}{Yet another summary}{-}{
    asjajsaj sakjskaj sjkajs
}

\subsection{varioref test}

\textbf{Varioref test}: Reference to \vref{test-id} and \vref{test-id-2},
\textbf{Pageref test}: \vpageref{test-id-2}

\subsection{Cref Test}

\cref{test-id}  \cpageref{test-id}

\subsection{etoolbox}

\makeatletter

A \csuse{test-id-2@Definition}

% How do I test for test-id-2 to bne defined?
B \ifdef{test-id-2}{\csuse{test-id-2@Definition}}{test-id-2 DOES NOT EXIST}

C \csuse{test-id-4@Definition}

D \ifdef{test-id-4}{\csuse{test-id-4@Definition}}{test-id-4 DOES NOT EXIST}

\makeatother

\subsection{zifrefundefined}

% Not robust against environments
\makeatletter
\zifrefundefined{test-id}{\nfss@text{\reset@font\bfseries??}}{
    \zref@extractdefault{#1}{Definition}{}
}
\makeatother

\end{document}

Answer 1

Markus, as far as I can see, you have two errors in your MWNE:

1. You have a typo in \zref@extractdefault{#1}{Definition}{}: I think #1 should be replaced by test-id
2. Probably a code reduction accident: You forgot to register your variables with \zref@labelbyprops, therefore zref appears as if it is not suitable for environments, but it is. So your assumption about zref is wrong.

Here is a MWE:

\documentclass[
  a4paper,
  11pt,
  parskip=half
]{scrbook}

\usepackage[nospace]{varioref}
\usepackage[pdfencoding=unicode, psdextra]{hyperref}
\hypersetup{linkcolor=blue}
\usepackage{cleveref}
\usepackage[user]{zref}
\usepackage{tcolorbox}

\tcbuselibrary{breakable}

\DeclareNewTOC[
  category=float,
  float,% declares floating environment eq
  floatpos=ht,
  nonfloat,% declares non-floating environment eq-
  listname={Some Items},
  name=TM,
  tocentrystyle=tocline,
  tocentrylevel:=table,
  tocentryindent:=table,
  tocentrynumwidth:=table,
  type=MySpecialEnvFloat,
]{ltm}

\newtcolorbox[
    auto counter,
    crefname={TM}{TMs}]{MyBox}[2][]{
    title={TM\,\thetcbcounter: #2},
    #1}
  

\makeatletter
\newcommand{\referToMySpecialEnv}[1]{
    \zifrefundefined{#1}{\nfss@text{\reset@font\bfseries??}}{
        \hyperref[#1]{\zref@extractdefault{#1}{ID}{}: 
        \emph{\zref@extractdefault{#1}{Definition}{}}} \vpageref{#1}
    }
}    
\makeatother
    
    
\begin{document}

\makeatletter
\zref@newprop{Definition}{}
\zref@newprop{ID}{}

\newcommand{\MySpecialEnv}[4]{%

    \begin{MyBox}[label={#1}]{#2}
        \zref@setcurrent{Definition}{#2}%
        \zref@setcurrent{ID}{TM\,\thetcbcounter}
        \zref@labelbyprops{#1}{Definition, ID}
        \addcontentsline{ltm}{section}{TM\,\thetcbcounter: #2}
        Some text
        \tcblower
        #4
    \end{MyBox}
}%  
\makeatother

\listofMySpecialEnvFloats

\MySpecialEnv{test-id}{Something that looks like a summary}{-}{
    Here is a cross-reference to
    % Works like a charm within the box!
    \referToMySpecialEnv{test-id-2}
}

\MySpecialEnv{test-id-2}{Yet another summary}{-}{
    Here is a cross-reference to
    % Works like a charm within the box!
    \referToMySpecialEnv{test-id}
}

\subsection{varioref test}

\textbf{Varioref test}: Reference to \vref{test-id} and \vref{test-id-2},
\textbf{Pageref test}: \vpageref{test-id-2}

\subsection{Cref Test}

\cref{test-id}  \cpageref{test-id}

\subsection{zifrefundefined}

% Works outside in the wild
Let us check \referToMySpecialEnv{test-id} and \referToMySpecialEnv{test-id-2}

\end{document}