I copied your code verbatiem adding only lines needed to check which path the script took,
However I couldnt reproduce the problem (mirc v6.2)

Code:
alias slap_manager_cb {
  if $2 == sclick {
    if $3 == 6 {
      xdid -t $1 6 Slap $iif($xdid($1,6).state,Checked,All)
    }
    elseif $3 == 1 {
      .scon $4
      echo -a $1-
      if !$5 {
        ///echo -a " if !$5 $true " | halt
        var %a = 1, %b = $chan(0)
        while %a <= %b {
          xdid -a $1 1 $+($4 %a,$chr(9),+e 0 0 0 1 0 $rgb(0,0,255) $chan(%a),$chr(9),)
          inc %a
        }
      }
      else {
        ///echo -a " if !$5 $false " | halt
        var %path = $xdid($1,1).selpath
        var %chan = $xdid($1,1,%path)
        echo 4 -a %path %chan
      }
    }
  }
  elseif !$istok(close changing sizing focus focusout,$2,32) && (mouse* !iswm $2) && (*activate !iswm $2) && (*mov* !iswm $2) {
    echo $color(info) -s */ slap_manager_cb: $1-
  }
}


//slap_manager_cb slap_manager sclick 1 1
slap_manager sclick 1 1
" if !$5 $true "

//slap_manager_cb slap_manager sclick 1 1 1
slap_manager sclick 1 1 1
" if !$5 $false "

//noop $slap_manager_cb(slap_manager,sclick,1,1)
slap_manager sclick 1 1
" if !$5 $true "

//noop $slap_manager_cb(slap_manager,sclick,1,1,1)
slap_manager sclick 1 1 1
" if !$5 $false "

all above were as expected,
nb: $5 being 0 or $null well result in a $true as im sure you know already, but below might be what your experencing, although it doesnt match to your problem with $5 being "1"

//echo $slap_manager_cb(slap_manager,sclick,1,1,$chr(32))
slap_manager sclick 1 1
" if !$5 $false "