Granted that the Senate sets all the important rules for the trial, John Roberts is still the presiding officer. I would have thought he has more authority than a court stenographer has. Since there’s no obvious reason for scheduling impeachment all the way to 2am, I would think nixing that would be something in his purview, however limited that purview is.
He's not a judge in a courtroom with all the power of a federal judge. He's temporary presiding officer of the Senate, in charge of enforcing Senate rules. The Senate calendar is under control of the majority leader who passed the rules of how the trial would be run. If the rules don't say "must adjourn for the day by X o'clock" then Roberts would not be allowed to do so on his own, he needs a Senator to ask for adjournment and then get consent from the rest of the Senate.