How to cite laws and regulations completely inline?

Since you want to use biblatex-juradiss as base I think it's the simplest to adjust the cite macro. To avoid interactions I'd define a new one.

[Edit: add loccit support & example]

documentclass{article}
usepackage[style=biblatex-juradiss,
sortcites=true,
sorting=none,
defernumbers=true,
minbibnames=3, % cite up to three authors in bib
backref=true,
giveninits=true,
loccittracker=constrict,
backend=biber]{biblatex}

% deutsch
usepackage[
english,
ngerman,
% shorthands=off
]{babel}
usepackage[ngerman]{isodate}usepackage[autostyle, german=quotes]{csquotes} % deutsche Anführungszeichen mit enquote

begin{filecontents}{jobname.bib}
    @commentary{djvIfgKommentierung,
        author = {Dr.~Wilhelm Mecklenburg Mecklenburg and Benno H. Pöppelmann},
        title = {Informationsfreiheitsgesetz},
        subtitle = {Gesetztexte, Kommentierungen, Fallbeispiele Erläuterungen},
        titleaddon = {Information – Ihr gutes Recht},
        date = {2007-02},
        editor = {{Deutscher Journalisten-Verband e.~V.}},
        editora = {{Vereinigte Dienstleistungsgewerkschaft (ver.di)}},
        editorb = {{Humanistische Union}},
        editorc = {{netzwerk recherche}},
        editord = {{Transparency International – Deutsches Chapter}},
        language = {ngerman},
        origlanguage = {ngerman},
        publisher = {DJV},
        location = {Berlin},%modified to use the real location field
        isbn = {978-3-935819-22-0},
        pagetotal = {223},
        DISurl = {https://netzwerkrecherche.org/wp-content/uploads/2014/07/ifg-kommentar-buch.pdf}
    }
end{filecontents}

addbibresource{jobname.bib}


DeclareLabeldate{%
    field{date}
    field{year}
    field{eventdate}
    field{origdate}
    field{urldate}
    literal{nodate}
}

ExecuteBibliographyOptions{labeldateparts}

%wrapper for the ibid citing
DeclareCiteCommand{legalcite}[mkbibbrackets]
{usebibmacro{prenote}}
{%
    ifthenelse{ifciteibidANDNOTiffirstonpage}%
    {%
        ifloccit{%
            globalbooltrue{cbx:loccit}%
            usebibmacro{cite:ibid}%
        }{%
            globalboolfalse{cbx:loccit}%
            usebibmacro{cite:loccit}}}%
    {usebibmacro{cite:legal}usebibmacro{cite:save}}%
}{multicitedelim}
{usebibmacro{cite:postnote}}


%cite format
newbibmacro*{cite:legal}{%
    usebibmacro{citeindex}%
    iffieldundef{shorthand}%
    {%
        ifnameundef{labelname} {} {%
            printnames[family-given]{labelname}%
            setunit{printdelim{nameyeardelim}}%
        }
        iffieldundef{labelyear} {} {
            usebibmacro{cite:labeldate+extradate}%
            setunit{addperiodaddspace}printdelim{yeartitledelim}}
        usebibmacro{cite:label}
        setunit{addperiodaddspace}
        usebibmacro{cite:location+publisher}%
    }%
    {usebibmacro{cite:shorthand}}%
}

%helper macros, this one is the definition of authoryear.cbx
newbibmacro*{cite:label}{%
    iffieldundef{label}
    {printtext[bibhyperref]{printfield[citetitle]{labeltitle}}}
    {printtext[bibhyperref]{printfield{label}}}}


newbibmacro*{cite:labeldate+extradate}{%
    iffieldundef{labelyear}
    {}
    {printtext[parens]{printlabeldateextra}}}

newbibmacro*{cite:location+publisher}{%
    iflistundef{location}{}{
        printlist{location}%
        iflistundef{publisher}
        {setunit*{addcommaspace}}
        {setunit*{addcolonspace}}%
    }%
    printlist{publisher}%
    setunit{addperiod}%
    newunit
}

%add loccite support, adopted from biblatex authortitle-ibid.cbx
newbibmacro*{cite:postnote}{%
    ifbool{cbx:loccit}%
    {}%
    {usebibmacro{postnote}}%
}

newbibmacro*{cite:loccit}{%
    printtext[bibhyperref]{bibstring[mkibid]{loccit}}}

AtBeginDocument{%to be changed after language setup
    renewcommand*{multinamedelim}{addcommaaddspace}
    renewcommand*{finalnamedelim}{addspace&addspace}
}

%In case you also want to get ibid, with empty postnote but that's overriding the test
%makeatletter
%defblx@loccit@numcheck#1{%
%   blx@imc@iffieldundef{postnote}%
%   {%
%       ifcsundef{blx@lastnote@#1@abx@field@entrykey}%
%           {@firstoftwo}%
%           {@secondoftwo}%
%   }%
%   {%
%       expandafterblx@imc@ifpages%
%       expandafter{abx@field@postnote}%
%       {blx@imc@iffieldequalcs{postnote}{blx@lastnote@#1@abx@field@entrykey}}%
%       {@secondoftwo}}}%
%ExplSyntaxOff
%makeatother


begin{document}
    
    section{Wanted (hardcoded)}
    Generell gilt, dass
    enquote{in der Begründung einer ablehnenden Entscheidung textelp{} die nachteiligen Auswirkungen konkret zu benennen und textendash darzulegen} [Mecklenburg, W. & Pöppelmann, B. (2007). Informationsfreiheitsgesetz. Berlin: DJV.] ist,
    enquote{warum die Möglichkeit besteht, dass solche textins{nachteilige} Auswirkungen textins{nach §,3 IFG Nr.~1} eintreten}[ebd.].
    
    
    begin{refsection}
        section{own cite macro Test}
        Generell gilt, dass
        enquote{in der Begründung einer ablehnenden Entscheidung textelp{} die nachteiligen Auswirkungen konkret zu benennen und textendash darzulegen}legalcite{djvIfgKommentierung} ist,
        enquote{warum die Möglichkeit besteht, dass solche textins{nachteilige} Auswirkungen textins{nach §,3 IFG Nr.~1} eintreten}legalcite{djvIfgKommentierung}.
    end{refsection}
    
    
    begin{refsection}
        legalcite[3]{djvIfgKommentierung} 
        
        legalcite[3]{djvIfgKommentierung}
        
        legalcite[4]{djvIfgKommentierung}
        
        legalcite{djvIfgKommentierung} 
        
        legalcite{djvIfgKommentierung} 
        
        
        legalcite[2]{djvIfgKommentierung} 
        
        
    end{refsection}
    
    
    printbibliography
    
end{document}

The name format is not setup correctly in you bibfile, so you might want to adjust this, but generally this is doing what you are expecting and compatible to the setup of biblatex-juradiss.