No 'et al.' for @collection @incollection

Question

I have recently stopped to use @inbook to make it do what @incollection is supposed to do. However, now I have the problem that it will not show every editor -- since my bibliography preamble at this point is quite big, I am not sure if I can change it myself with the other questions concerning 'et al.'. My attempts thus far have failed... so here's the MWE:

\documentclass[paper=a4,12pt,numbers=endperiod]{scrartcl}
\usepackage[backend=biber,style=authoryear]{biblatex}
\addbibresource{TransAlph.bib}
\DefineBibliographyStrings{english}{references={Bibliography}}
\DeclareFieldFormat{editortype}{\mkbibparens{#1}}
\renewbibmacro{byeditor+others}{%
    \ifnameundef{editor}
    {}
    {\printnames[byeditor]{editor}%
        \setunit{\addspace}%
        \usebibmacro{byeditor+othersstrg}%
        \clearname{editor}%
        \newunit}%
    \usebibmacro{byeditorx}%
    \usebibmacro{bytranslator+others}}
\renewbibmacro{byeditor+othersstrg}{\usebibmacro{editor+othersstrg}}
\renewbibmacro{bytranslator+othersstrg}{\usebibmacro{translator+othersstrg}}
\renewbibmacro{bytypestrg}[2]{%
    \iffieldundef{#1type}
    {\bibstring{#2}}
    {\ifbibxstring{\thefield{#1type}}
        {\bibstring{\thefield{#1type}}}
        {\printtext{\thefield{#1type}}}}}
\renewcommand{\multinamedelim}{/}
\renewcommand{\finalnamedelim}{\multinamedelim}
\renewcommand{\labelnamepunct}{\addcolon\space}
\renewcommand{\postnotedelim}{\addcolon\space}
\DeclareFieldFormat{postnote}{#1}
\DeclareFieldFormat{multipostnote}{#1}
\DeclareFieldFormat{superedition}{\textsuperscript{#1}}
\providecommand*{\iflabeldateisdate}{%
    \ifboolexpr{%
        not test {\iffieldundef{labeldatesource}}
        and 
        (test {\iffieldequalstr{labeldatesource}{}} 
        or test {\iffieldequalstr{labeldatesource}{year}})}}
\renewbibmacro{date+extrayear}{%
    \iffieldundef{labelyear}
    {}
    {\usebibmacro{origdate}%
        \setunit{\addspace}%
        \printtext[parens]{%
            \iflabeldateisdate
            {\iffieldnum{edition}
                {\printfield[superedition]{edition}%
                    \global\clearfield{edition}}
                {}%
                \printdateextra}
            {\printlabeldateextra}}}}
\newbibmacro*{origdate}{%
    \iffieldundef{origyear}
    {}
    {\printtext[brackets]{\printorigdate}}}
\renewcommand*{\bibpagespunct}{\addcolon\space}
\DeclareFieldFormat{pages}{#1}
\DeclareNameAlias{sortname}{family-given}
\DeclareFieldFormat[article,periodical]{number}{\mkbibparens{#1}}
\renewbibmacro{volume+number+eid}{%
    \printfield{volume}%
    %\setunit{\adddot}%<- comment this
    \printfield{number}%
    \setunit{\addcomma\space}%
    \printfield{eid}}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Collection{dahmen1991a,
Title                    = {Zum Stand der Ko-di-fi-zie-rung ro-ma-ni-scher Klein-spra-chen. Ro-ma-nis-ti-sches Kol-lo-qui-um,V \textup{(Tü-bin-ger Bei-trä-ge zur Lin-gu-is-tik,348)}},
Editor                   = {Wolf-gang Dah-men and Ot-to Gsell and Gün-ter Hol-tus and Jo-han-nes Kra-mer and Mi-cha-el Met-zel-tin and Ot-to Win-kel-mann},
Keywords                 = {sek},
Location                 = {Tü-bin-gen},
Publisher                = {Gun-ter Narr},
Year                     = {1991}
}
@Incollection{kaislaniemi2017a,
Title                    = {`A Gra-phic Sys-tem which Leads its own Lin-gu-is-tic Life?' Epis-to-la-ry Spel-ling in En-glish, 1400--1800},
Author                   = {Kai-sla-nie-mi, Sa-mu-li and Evans, Mel and Ju-vo-nen, Teo and Sai-rio, An-ni},
Booktitle                = {Säi-ly/Nur-mi/Pa-lan-der-Col-lin/Au-er, (eds.) (2017)},
Keywords                 = {sek},
Pages                    = {187--213},
Year                     = {2017}
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
Example sentence \textcite[cf.][3--6]{kaislaniemi2017a}.
Another example sentence \parencite[][39]{dahmen1991a}.
\printbibliography
\end{document}

Thank you so much!

Edit: I would like the output to be:

Dahmen, Wolfgang/Gsell, Otto/Holtus, Günter/Kramer, Johannes/Metzeltin, Michael/Winkelmann, Otto, (eds.) (1991): Zum Stand der Kodifizierung romanischer Kleinsprachen. Romanistisches Kolloquium V (Tübinger Beiträge zur Linguistik 348). Tübingen: Gunter Narr.

Kaislaniemi, Samuli/Evans, Mel/Juvonen, Teo/Sairio, Anni (2017): “‘A Graphic System which Leads its own Linguistic Life?’ Epistolary Spelling in English, 1400–1800”. In: Säily/Nurmi/Palander-Collin/Auer, (eds.) (2017): 187–213.

with "Zum Stand der Kodifizierung romanischer Kleinsprachen. Romanistisches Kolloquium V" and "Säily/Nurmi/Palander-Collin/Auer, (eds.) (2017)" in cursive.

Answer 1

In addition to following @moewe's suggestion and adding the option maxbibnames=20 (or some other suitably chosen large number) while loading biblatex, you should also change

\renewcommand*{\multinamedelim}{/}

to either

\renewcommand*{\multinamedelim}{\slash}

or, better still,

\DeclareDelimFormat{multinamedelim}{\slash}

This will allow line breaks after the / symbols. As @moewe points out in a comment, the second option is preferable since it allows for context-sensitive changes the the name delimiter symbol.

If you need to fine-tune the hyphenation patterns, use \hyphenation commands in the preamble; don't clutter the bibliographic entries with \- discretionary hyphenation points. In the example below, I used \hyphenation{klein-spra-chen tü-bin-gen} to make sure that LaTeX would perform the correct hyphenation operation on both words.

Do also consider loading the csquotes package and using its \enquote macro to "quote" material, say, the first part of the title string, in smart single-quote characters.

Last but certainly not least, you should make (much more of) a concerted effort to provide correct and usable information in the bib entries. For sure, this is how I would set up the two entries in question:

@Collection{dahmen1991a,
Title     = {Zum Stand der Kodifizierung romanischer Kleinsprachen. Romanistisches Kolloquium\,V},
series    = {Tübinger Beiträge zur Linguistik},
number    = {348},
Editor    = {Wolfgang Dahmen and Otto Gsell and Günter Holtus and Johannes Kramer and Michael Metzeltin and Otto Winkelmann},
Keywords  = {sek},
Location  = {Tübingen},
Publisher = {Gunter Narr},
Year      = {1991},
}
@Incollection{kaislaniemi2017a,
Title     = {\enquote{A Graphic System which Leads its own Linguistic Life?} Epistolary Spelling in English, 1400--1800},
Author    = {Kaislaniemi, Samuli and Evans, Mel and Juvonen, Teo and Sairio, Anni},
Editor    = {Tanja Säily and Arja Nurmi and Minna Palander-Collin and Anita Auer},
Booktitle = {Exploring Future Paths for Historical Sociolinguistics},
Series    = {Advances in Historical Sociolinguistics},
Number    = 7,
Keywords  = {sek},
Pages     = {187--213},
Year      = {2017},
Location  = {Amsterdam},
Publisher = {John Benjamins},
}

Note that I added the fields series and number in both entries (backfilling data as needed), and I seriously revamped the editor field and added an entirely new booktitle field for the kaislaniemi2017a entry.