So... why can't the pushback and start be done with INT selected (i.e. hot mike) to the guy on headset? As I said above, this incident almost certainly wouldn't have happened had that been the case as the guy downstairs would have piped up when they started talking about the taxi route or even earlier reading checklists.
Keeping him (be he LAME, casual rampie or bag chucker) in the loop during the start seems to me to be a completely logical and safe operational practice. You've got two people working in sync who can't see each other but are reliant on each other to achieve the task and you deliberately cut one out? Where's the sense in that?