How to define a comma separated list of arguments for includegraphics

Just define new keys:

\documentclass{article}
\usepackage{graphicx}

\makeatletter
\define@key{Gin}{settingsA}[]{\setkeys{Gin}{width=\textwidth, angle=90}}
\define@key{Gin}{settingsB}[]{\setkeys{Gin}{page=1, scale=.1}}
\define@key{Gin}{settingsC}[]{\setkeys{Gin}{angle=-90, scale=.5}}
\makeatother

\begin{document}

\includegraphics[settingsA]{example-image-a}

\includegraphics[settingsB]{example-image-b}

\includegraphics[settingsC]{example-image-c}

\end{document}

A bit of explanation. The graphicx package relies on keyval provided key-value syntax and reserves Gin (Graphic inclusion) as the family name of the keys for \includegraphics. A new key is simply added by

\define@key{Gin}{<name>}[<default value>]{<code>}

The \includegraphics[<options>]{file} command will do

\setkeys{Gin}{<options>}

and every known key will be processed. The default empty value given for settingsA and so on is so that you can simply type settingsA without =something.

One could even improve it; say that in settingsC you want to be able to vary the angle; with

\define@key{Gin}{settingsC}[-90]{\setkeys{Gin}{angle=#1, scale=.5}}

you can specify settingsC for the angle to be -90 or

settingsC=45

for the angle to be 45.

Caveat Be sure not to define an already existing key: \define@key would silently overwrite it.

Make your own include graphics command. Something like the following:

\newcommand\myincludegraphics[2]{%%
   \expandafter\includegraphics\expandafter[#1]{#2}}

The problem with your code is that in the construction

\includegraphics[\settingsA]{filehandle}

The unexpanded \settingsA is getting passed to the key parser where it will fail to match any keys. By using the above command, you can write something like,

\myincludegraphics{\settingsA}{filehandle}

By using \expandafter, \settingsA will be expanded by the time \includegraphics has a chance to pass the optional arguments to its parser.