(Continues.)

Replace the body code in JB1 (but not in JB2) by:

 {\IfNoValueTF{#4}
   {\let\thecontentslabel\@empty
    \ttl@numsecfalse}
   {\gdef\thecontentslabel{#4}
    \ttl@numsectrue}
  \gdef\thecontentspage{#7}
  \DoParameterAssignments
  \global\let\COpost@action\COpost@action
  \global\let\COobject@decl\COobject@decl
  \global\@tempdimd\@tempdimd
  \global\@tempdime\@tempdime
  \global\@tempdimb\@tempdimb
  \global\let\COnumber@format\COnumber@format
  \global\@tempdima\@tempdima
  \global\let\COtitle@format\COtitle@format
  \global\let\COtitle@format@nolbl\COtitle@format@nolbl
  \global\let\COpnum@format\COpnum@format
  \global\let\COleaders@action\COleaders@action
  \global\@tempdimc\@tempdimc
  \ttl@tocentry
   {\@ne}
   {#3}
   {\@tempdimd}
   {\contentsmargin[\@tempdimb]{\@tempdime}
    \COpre@action
    \COobject@decl}
   {
     {\contentslabel[\COnumber@format]{\@tempdima}\COtitle@format}
     {\hspace*{-\@tempdima}\COtitle@format@nolbl}
   }
   {\titlerule*[\@tempdimc]{\COleaders@action}
    \contentspage[\COpnum@format{\thecontentspage}]}
   {\COpost@action}
   {#5}
   {}}

As you can see, the code is the same but with a series of global
assignments right after \DoParameterAssignments. Here is an example:

\DeclareInstance{contentsobject}{section}{JB1}{
  pre-action       = \DelayEvaluation{\addvspace{1pc plus 1pt}},
  right-adjust     = 3pt,
  title-format     = \textbf{#1},
  label-format     = \textbf{\thecontentslabel},
  left-margin      = \DelayEvaluation{2.5em},
  label-width      = \DelayEvaluation{2.5em},
  page-format      = \textbf{\hfil#1},
  leaders-text     = }


\DeclareInstance{contentsobject}{subsection}{JB2}{
    pre-action     = \addvspace{3pt plus .2pt},
    decls          = \itshape,
    left-margin    = \DelayEvaluation{2.5em},
    punct-sep      = {;\ },
    punct-end      = .
    }

\bye
Javier