So there seems to be so much wrong with the TargetGroup.
I have tried both ways that you suggested and while the list is updated in both cases the TargetGroup fails to update it's internal focusIndex.
So after I have deleted all of the content nodes and replaced them the focus could still be on item 3 (or any greater than 0).
If I call the below code the UI updates to show the first item of the list is now focused but if you try to use the arrows left and right the UI updates as if the old focusIndex was still set.
m.titlesList.animateToItem = 0