How to pass new values to keys without overwriting other values

Motivation

I’m working on a custom Paragraph command, intended for use inside align-type environments.

Code

Here it is. It currently performs as desired, but see Problem below.

documentclass{article}
usepackage{xparse,amsmath}

NewDocumentCommand{Aligned}{O{t} m}{
    begin{aligned}[#1]#2end{aligned}
}

ExplSyntaxOn

% define keys
keys_define:nn { Paragraph }{
    beforeparagraph  .tl_set:N = l__Paragraph_beforeparagraph_tl,
    beforehead  .tl_set:N = l__Paragraph_beforehead_tl,
    width .tl_set:N = l__Paragraph_width_tl,
    afterhead .tl_set:N = l__Paragraph_afterhead_tl,
    beforebody .tl_set:N = l__Paragraph_beforebody_tl,
    bodyindent .tl_set:N = l__Paragraph_bodyindent_tl,
    afterparagraph .tl_set:N = l__Paragraph_afterparagraph_tl,
}

cs_new_protected:Nn Paragraph_beforeparagraph:n{#1}
cs_new_protected:Nn Paragraph_beforehead:n{#1}
cs_new_protected:Nn Paragraph_width:n{#1}
cs_new_protected:Nn Paragraph_afterhead:n{#1}
cs_new_protected:Nn Paragraph_beforebody:n{#1}
cs_new_protected:Nn Paragraph_bodyindent:n{#1}
cs_new_protected:Nn Paragraph_afterparagraph:n{#1}
cs_generate_variant:Nn __Paragraph_build:nnnnnnnnn {VVVnVVVnV}

% auxiliary function to avoid expansion problems and do some of the formatting
cs_new_protected:Nn __Paragraph_build:nnnnnnnnn {
    Paragraph_beforeparagraph:n { #1 }
    Aligned{
        tl_if_empty:nF { #4 }{
            Paragraph_beforehead:n { #2 }
            tl_if_empty:nTF { #3 }{text{#3}}{parbox{#3}{#4}}
            Paragraph_afterhead:n { #5 }
        }
        tl_if_empty:nF { #8 }{
            Paragraph_beforebody:n { #6 }
            Paragraph_bodyindent:n { hspace{#7} }
            { Aligned{#8} }
        }
    }
    Paragraph_afterparagraph:n { #9 }
}

% formatting
NewDocumentCommand{Paragraph}{
    O{
        beforeparagraph={&},
        beforehead={&},
        width=10cm, 
        afterhead={},
        beforebody={&},
        bodyindent=1em,
        afterparagraph={}
    } m m
    }{
    keys_set:nn { Paragraph } { #1 } % populate keys
    % format paragraph
    __Paragraph_build:VVVnVVVnV
        l__Paragraph_beforeparagraph_tl
        l__Paragraph_beforehead_tl
        l__Paragraph_width_tl
        { #2 }
        l__Paragraph_afterhead_tl
        l__Paragraph_beforebody_tl
        l__Paragraph_bodyindent_tl
        { #3 }
        l__Paragraph_afterparagraph_tl
}

ExplSyntaxOff

Problem

How can I change one of the values without changing all of them?

begin{document}
    begin{align*}
        Paragraph{the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog}{
             2^2 + 3^2
             & = 4 + 9 
             & = 13
        } % example with default options
        Paragraph[width=8cm]{the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog}{1+1=2} % should overwrite only width; actually overwrites all optional parameters
    end{align*}
end{document}

documentclass{article}
usepackage{xparse,amsmath}

ExplSyntaxOn

NewDocumentCommand{Aligned}{O{t} m}
  {
    begin{aligned}[#1]#2end{aligned}
  }


% define keys
keys_define:nn { Paragraph }
  {
    beforeparagraph .tl_set:N  = l__Paragraph_beforeparagraph_tl,
    beforehead      .tl_set:N  = l__Paragraph_beforehead_tl,
    width           .tl_set:N  = l__Paragraph_width_tl,
    afterhead       .tl_set:N  = l__Paragraph_afterhead_tl,
    beforebody      .tl_set:N  = l__Paragraph_beforebody_tl,
    bodyindent      .tl_set:N  = l__Paragraph_bodyindent_tl,
    afterparagraph  .tl_set:N  = l__Paragraph_afterparagraph_tl,
  }

cs_new_protected:Nn Paragraph_beforeparagraph:n{#1}
cs_new_protected:Nn Paragraph_beforehead:n{#1}
cs_new_protected:Nn Paragraph_width:n{#1}
cs_new_protected:Nn Paragraph_afterhead:n{#1}
cs_new_protected:Nn Paragraph_beforebody:n{#1}
cs_new_protected:Nn Paragraph_bodyindent:n{#1}
cs_new_protected:Nn Paragraph_afterparagraph:n{#1}
cs_generate_variant:Nn __Paragraph_build:nnnnnnnnn {VVVnVVVnV}

% auxiliary function to avoid expansion problems and do some of the formatting
cs_new_protected:Nn __Paragraph_build:nnnnnnnnn
  {
    Paragraph_beforeparagraph:n { #1 }
    Aligned
      {
        tl_if_empty:nF { #4 }
          {
            Paragraph_beforehead:n { #2 }
            tl_if_empty:nTF { #3 }{text{#3}}{parbox{#3}{#4}}
            Paragraph_afterhead:n { #5 }
          }
        tl_if_empty:nF { #8 }
          {
            Paragraph_beforebody:n { #6 }
            Paragraph_bodyindent:n { hspace{#7} }
            { Aligned{#8} }
          }
      }
    Paragraph_afterparagraph:n { #9 }
  }

% formatting
NewDocumentCommand{Paragraph}{ O{} m m }
  {
    keys_set:nn { Paragraph }
     {
       beforeparagraph = {&},
       beforehead      = {&},
       width           = 10cm, 
       afterhead       = {},
       beforebody      = {&},
       bodyindent      = 1em,
       afterparagraph  = {},
       #1
     } % populate keys
    % format paragraph
    __Paragraph_build:VVVnVVVnV
        l__Paragraph_beforeparagraph_tl
        l__Paragraph_beforehead_tl
        l__Paragraph_width_tl
        { #2 }
        l__Paragraph_afterhead_tl
        l__Paragraph_beforebody_tl
        l__Paragraph_bodyindent_tl
        { #3 }
        l__Paragraph_afterparagraph_tl
  }

ExplSyntaxOff

begin{document}

begin{align*}
Paragraph{
  the quick brown fox jumps over the lazy dog the quick 
  brown fox jumps over the lazy dog}{
  2^2 + 3^2
  & = 4 + 9 
  & = 13
}
Paragraph[width=8cm]{
  the quick brown fox jumps over the lazy dog 
  the quick brown fox jumps over the lazy dog}{1+1=2} 
end{align*}

end{document}