I have to call total bullshit on the assistant's passport situation. No way would Michael have not realized that he needed a new passport, as he was making all the maestro's travel arrangements. And even if he really is that stupid, he would have been able to get an emergency one done within 24 hours in New York City. So, presuming that this was at least a 10-day tour, he could have caught up with the orchestra after the first concert easily.
But, to paraphrase a saying from the TWoP days -- or was it Mighty Big TV? -- contrivance was getting a Coke from the fridge.