Secondly I'm drawing a selection around the text (then filling with black). There seems to be no way to centralise a selection box either. As when I run the action on different sized photos the box and text end up in completely different places, separately. Why is this and how can I centralise a selection too?
You can now align all three.
Type the text and position it.
Using the Rectangle tool (with "Shape layers" selected in the Options bar, top left) draw the box with desired colour as the foreground colour. Drag this new layer beneath the text layer; then select that layer, plus the text layer and do Ctrl+A.
Align.
Edit: You realised! :)