Try combining the following macro with a change in your TOC field, from:
"Heading 2,2,Heading 3,3,ATA MEL Item,5"
to:
"Heading 2,2,Heading 3,3" \f
Code:
Sub Demo()
Application.ScreenUpdating = False
Dim Tbl As Table, r As Long, Rng As Range, StrTC As String
For Each Tbl In ActiveDocument.Tables
With Tbl
If Split(.Cell(1, 1).Range.Text, vbCr)(0) = "ATA - SYSTEM &" Then
For r = 5 To .Rows.Count
StrTC = "TC " & Chr(34) & Split(.Cell(r, 1).Range.Text, vbCr)(0)
Set Rng = .Cell(r, 2).Range
With Rng
.End = .End - 1
StrTC = StrTC & " " & .Text & Chr(34) & " \l 5"
.Collapse wdCollapseEnd
.Fields.Add .Duplicate, wdFieldEmpty, StrTC, False
End With
Next
End If
End With
Next
Application.ScreenUpdating = True
End Sub