The plane Andrew, Edward, Sophie and William were on was originally scheduled to leave at 1.30pm (this comes from the flight manifesto). It was then delayed for an hour and ten minutes, meaning it took off at 2.39pm. The flight took 1 hour and 10 minutes, so they landed at Aberdeen at 3.50pm, And yes, then had to drive for at least an hour to get to Balmoral.
So, even in a best case scenario, they would have landed at Aberdeen at around 2.40pm, and not had time to drive to Balmoral before QEII’s death.
The weather was pretty bad that afternoon however, and so a helicopter transfer from Aberdeen wasn’t possible. We know it had been clearer earlier in the day because Charles had flown in by helicopter from Dumfries to Balmoral. Who knows if they’d been able to get there in time if it had still been clear enough between 2.30 and 3pm to fly?