For all bibliography items containing multiple editors, I must print the same order of names for multiple editors as I have them for multiple authors. For the authors, I got it to work (see here), but this does not cover the case of multiple editors (unless I made a mistake?).
The relevant part of a representative bibliography item currently looks like
G. E. R. Lloyd/G. E. L. Owen (ed. by)
but it must be
Lloyd, G. E. R./Owen, G. E. L. (ed. by)
MWE
\documentclass[
12pt,
a4paper
]
{scrreprt}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
%\usepackage[english]{babel}
\usepackage[
language=auto,
style=authoryear,
backend=bibtex,
isbn=false,
doi=false,
citereset=chapter,
maxcitenames=3,
dashed=false,
sorting=nyt,
firstinits=true,
terseinits=false
maxbibnames=99,
uniquename=init
]{biblatex}
\renewbibmacro*{byeditor+others}{%
\ifnameundef{editor}
{}
{\printnames[byeditor]{editor}%
\setunit{\addspace}%
\usebibmacro{byeditor+othersstrg}%
\clearname{editor}%
\newunit}%
\usebibmacro{byeditorx}%
\usebibmacro{bytranslator+others}}
\usepackage{xpatch}
\xpatchbibmacro{byeditor+othersstrg}{\printtext}{\printtext[parens]}{}{}
\begin{filecontents*}{lit.bib}
@InCollection{brandt,
options = {useprefix=false},
hyphenation = {german},
indexsorttitle = {Nordischen Lander von der Mitte des 11. Jahrhunderts bis 1448},
author = {von Brandt, Ahasver and Erich Hoffmann},
editor = {Ferdinand Seibt},
indextitle = {Nordischen L{\"a}nder von der Mitte des 11.~Jahrhunderts bis 1448, Die},
title = {Die nordischen L{\"a}nder von der Mitte des 11.~Jahrhunderts bis 1448},
shorttitle = {Die nordischen L{\"a}nder},
booktitle = {Europa im Hoch- und Sp{\"a}tmittelalter},
series = {Handbuch der europ{\"a}ischen Geschichte},
number = {2},
publisher = {Klett-Cotta},
location = {Stuttgart},
date = {1987},
pages = {884--917},
annotation = {An \texttt{incollection} entry with a \texttt{series} and a \texttt{number}.
Note the format of the printed name and compare the \texttt{useprefix} option in
the \texttt{options} field as well as \texttt{vangennep}. Also note the
\texttt{indextitle, and \texttt{indexsorttitle} fields}}
}
@InProceedings{moraux,
keywords = {secondary},
hyphenation = {french},
indexsorttitle = {De Anima dans la tradition grecque},
author = {Moraux, Paul},
editor = {Lloyd, G. E. R. and Owen, G. E. L.},
indextitle = {\emph{De Anima} dans la tradition gr{\`e}cque, Le},
title = {Le \emph{De Anima} dans la tradition gr{\`e}cque},
subtitle = {Quelques aspects de l'interpretation du trait{\'e}, de Theophraste {\`a}
Themistius},
shorttitle = {\emph{De Anima} dans la tradition gr{\`e}cque},
booktitle = {Aristotle on Mind and the Senses},
booktitleaddon = {Proceedings of the Seventh Symposium Aristotelicum},
eventdate = {1975},
publisher = cup,
location = {Cambridge},
date = {1979},
pages = {281--324},
annotation = {This is a typical \texttt{inproceedings} entry. Note the \texttt{booksubtitle},
\texttt{shorttitle}, \texttt{indextitle}, and \texttt{indexsorttitle} fields.
Also note the \texttt{eventdate} field.}
}
\end{filecontents*}
\addbibresource{lit.bib}
\DeclareNameAlias{sortname}{last-first}
\renewcommand{\multinamedelim}{\addslash}
\renewcommand{\finalnamedelim}{\addslash}
\begin{document}
\chapter{First Chapter}
\begin{itemize}
\item For the case of multiple editors, these are just some words.\footcite[Cp.][p. xi]{moraux}. This currently looks like
\begin{verbatim}
G. E. R. Lloyd/G. E. L. Owen (ed. by)
\end{verbatim}
but I must have
\begin{verbatim}
Lloyd, G. E. R./Owen, G. E. L. (ed. by)
\end{verbatim}
\end{itemize}
\printbibliography
\end{document}
