(Stupid me. Yesterday I overlooked that when it's only about suppressing and not suppressing expansion,  the order in which tokens get processed doesn't matter. Thus the following is sufficient:)

Perhaps instead of preventing the carrying-out of referencing-commands and preventing the change to uppercase/lowercase of rererencing-commands' arguments within  \tl_upper_case:n /  \tl_lower_case:n, referencing-commands and the like can be redefined within \tl_upper_case:n /  \tl_lower_case:n to do all expansion-work possible (gathering textual phrases or questionmarks) before reaching a level w…

\DeclareRobustCommand\sometext{Hallo}
\section{title in lowercase \sometext  }

\usepackage{ducksay}
% \usepackage{siunitx}% Throws Undefined control sequence \textasciiacute instead
% \usepackage{textcomp}% Works
\AddAnimal{whale}
{ \                  '-.     '
   \      .---._      \ \.--''
    \   /       `-. .__)  ,-'
     \ |     .           /
        \--.__,   . __.,´
         `-.___'._\ _.'       }
\begin{document}
\ducksay[whale]{! Package inputenc Error: Unicode character (U+B4)}
\end{document}

\documentclass[]{article}
\usepackage{ducksay}
\AddAnimal{whale}
{ \                 '-.     '
   \     .---._      \ \.--''
    \  /       `-. .__)  ,-'
      |     .           /
       \--.__,   . __.,'
        `-.___'._\ _.'       }

\begin{document}
\ducksay[whale]{This doesn't work}
\end{document}

\documentclass{article}
\usepackage[check-declarations]{expl3}

\ExplSyntaxOn
\newcommand\test{ \seq_gset_from_clist:Nn \l_tmpa_seq {1,2,3,4} }
\ExplSyntaxOff


\begin{document}

\test

\end{document}

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