Primary position

Yep, so I ask trainees to consider their visibility, car doors, oncoming traffic, effect on other road users, road surface and room for manoeuvre. Not instruct 'ride in this default primary position'.

Maybe I'm really wondering about where to pitch the balance between instructing and coaching?

there must be a blend of both those things, to me instructing is letting people know the rules etc, the framework for what youre teaching them, and the coaching is seeing them doing it, and advising as they do it.
and within the instructing youre doing- you will be moving between roles too,
so whilst explaining the decision making needed in risk assessing their riding, youre questioning, drawing it out of them, there might be flexibility needed from both sides in establishing the best consensus, so its a facilitative role.
and if youre at a junction telling them where they definately dont want to be riding- no discussion- then your autocratic, you need to be sometimes, in a good way of course.