Wherever the feet eventually end up as their precise ideal spot, they certainly won't be anywhere near both on the shot line where they are in the photo so you can begin the feet adjustment and fine tune for comfortable head placement if the student doesn't just find it automatically when they open up...which they very well might given that a standard parallel step in will accommodate quite a few visual centers from a very similar foot placement. Even if their visual center isn't ideally set up, we aren't talking about changing by a whole lot to get it right at that point. That's what makes it FINE tuning. Before we get to that, we have to take out the most glaring obvious problem in the setup by stepping in right to give the arm some clearance over the shot line thereby making the bulk of the adjustment in one step.
I agree with you about proper head placement being an absolute must. But this doesn't need to be found if the student is already doing it. By getting him to open up and undo the crowding of the line that is causing the elbow flare, we aren't necessarily pulling the head position off the spot it wants to be in over the cue.
Really we are nitpicking here about order. No matter what, these two things have to happen very early on as without opening up, the player will still be crowding the line, and without having his head in the right spot to see shots correctly once open, his improved mechanics won't help him much if he can't see the shot right. We need both.
FWIW, DMing with Pegas recently (before this discussion even), he too determines the visual center and builds everything around that. You two aren't as far apart as you might think