To create a separator line between heading and items, you can use yet another label. Do not add any text to that label, but set its "Border" to the desired color (e.g. black), its "Insets" to "0 0 0 0", its "Placement" to "Internal: Center", and its "Size" to "Fit Node Width".
Corresponding GraphML.
The second label can be moved the same way as all other labels: Select it (press and hold CTRL, then left-click) and either drag it to the desired position or choose an appropriate "Placement". However, in an oval node, only horizontally centered positions will work well.