Hello @KremeChoco,
Thank you for the very detailed post information.
Please be advised that any element added inside a column will only display within the bounds of the columns. There is no way you can insert a line element inside a column hoping that it would spill over towards the other columns.
What you can do is to insert a gap element in the succeeding columns and make sure that the gap element has the same height as the headline element. You will also have to insert line element in each of the columns instead.
Alternatively, you can separately add the headline and line element in a one column row. You can then set this row to have a negative bottom margin so that it displays overlapped with the three columns thus the line will pass through the 3 columns.
Hope this make sense.