Custom Align Environment – Define and Align Starred Version

Question

I would like to redefine both the align and \align* environments such that the first line is shifted up to line up with the list identifier. I attempted to adapt the solution provided at Starred equivalent in newenvironment, along with \NewDocumentEnvironment from xparse but am unable to get the Ealign* environment to work. Commenting out the last \item in the list yields: LaTeX Error: Environment Ealign* undefined..

\documentclass{article}
\usepackage[fleqn]{amsmath}
\usepackage[shortlabels]{enumitem}
\usepackage{xparse}

\NewDocumentEnvironment{Ealign}{s}{%
    \IfBooleanTF {#1}{%
        \csname align*\endcsname\\[-8.6ex]%
    }{%
        \csname align\endcsname\notag\\[-8.6ex]%
    }%
}{%
    \IfBooleanTF {#1}{%
        \csname endalign*\endcsname%
    }{%
        \csname endalign\endcsname%
    }%
}%


\begin{document}
\begin{enumerate}[(a)]
\item
\begin{align}
    a &= b\\
    c &= d
\end{align}
%
\item
\begin{Ealign}
    a &= b\\
    c &= d
\end{Ealign}
\hrule%--------------------------
\item
\begin{align*}
    a &= b\\
    c &= d
\end{align*}
% Following yields: ! LaTeX Error: Environment Ealign* undefined.
%\item
%\begin{Ealign*}
%    a &= b\\
%    c &= d
%\end{Ealign*}
\end{enumerate}
\end{document}

As an aside not, this question about Vertical alignment of align* in enumerate is directly related to the issue I am trying to solve here. While that one provides what seems to be a pretty complicated, but automated method of correcting the alignment, it does not address defining a new enviroment for both the \align* and \align. I will probably try to incorporate this into the Ealign environment and eliminate my hackish solution of using \\[-8.6ex], which does not work when the align contains things such as fractions (so would require manually specifying the space value).

Answer 1

I cannot see the sense of a hardcoded vertical space. However, without xparse try:

\documentclass{article}
    
\usepackage[fleqn]{amsmath}
    
\usepackage[shortlabels]{enumitem}
    
\makeatletter
    
\newenvironment{Ealign}
      
  {\par\vspace{-8.6ex}
\align}
      
  {\endalign}

\newenvironment{Ealign*}
  {\par\vspace{-8.6ex}
      
   \start@align\@ne\st@rredtrue\m@ne
}
  {\endalign
}

    \makeatother
    
    
\begin{document}
...