Customizing inline enumerate with enumitem still not working?

Nothing can say it better than the manual.

The three inline lists have types enumerate*, itemize*, and description*, which are available always, even without the package option inline (which just defines three environments with these names).

The starred form \setlist* adds the settings to previous ones.

You should create a \newlist instead and there is no more need to put the inline option unless you want to use the predefined lists.

\documentclass{article}
\usepackage{enumitem}

\setlist[enumerate,1]{%
  label=\arabic*.,
}

\newlist{inlinelist}{enumerate*}{1}
\setlist*[inlinelist,1]{%
  label=(\roman*),
}

\begin{document}

\begin{enumerate}
  \item item 1
  \item item 2
  \item item 3
\end{enumerate}

\begin{inlinelist}
  \item item 1
  \item item 2
  \item item 3
\end{inlinelist}

\end{document}

Easy control using "shortlabels" option

\documentclass{article}
\usepackage[inline,shortlabels]{enumitem}


\begin{document}

\begin{enumerate}[1.]
  \item item 1
  \item item 2
  \item item 3
\end{enumerate}

\begin{enumerate*}[(i)]
  \item item 1
  \item item 2
  \item item 3
\end{enumerate*}

\end{document}