question

Edit: I wanted to answer you quickly before, but I didn't have time for a complete answer. I will elaborate.

Part 1:
You can insert a Derived Columns transformation in your error path before your error destination. Then add new columns from user and system variables or other expressions. Finally, map the new column(s) in your error destination.

Part 2:
Your suspicion is correct. The MaximumErrorCount property is in reference to container errors. If you redirect row errors in a data flow, you have successfully handled them so the container (the data flow) finishes successfully. Consider the property to mean Maximum**Unhandled**ErrorCount. You can insert a Row Count transformation to count how many rows are redirected. If you don't want to log the errors if the count exceeds a limit, this is what I suggest:

1. Create a new MaxErrorRowsCount package variable.
2. Add the script tranformation to the error path with the following code: (I had to manually format with HTML to get it to display correctly.)

Public Overrides Sub Input0_ProcessInput(ByVal Buffer As Input0Buffer)
    Dim MyRowCount As Integer = 0
    While Buffer.NextRow()
        'Input0_ProcessInputRow(Buffer)
        MyRowCount = MyRowCount + 1
        If MyRowCount > Variables.MaxErrorRowsCount Then
            Throw New Exception()
        End If
    End While
End Sub

1. Put your original error destination after the script.

If there are too many errors, the task (and package) will fail. If not too many, they will be logged. I tested it, and it worked for me.