In fact, if Roberts doesn't show, he's going to create a Consitutional crisis because Trump's lawyers will immediately file for a permanent injunction/dismissal with prejudice of any attempt to hold a Senate trial without Roberts, and HE will have to rule on HIS OWN ABSENCE.
Chief Justice only presides in trial of "the President", meaning the sitting President....text doesn't say "a President". Senate trial is under Senate rules, so there is no appellate authority beyond the chair, which in this case will be VP Harris.