So, when Leakey Leahey presides over the case where he will also be sitting as a juror, that’s not OK, either. Right?
Since Trump isn’t the sitting President, Roberts was not needed.
Leahy is president pro tempore of the Senate which is why he is presiding as the judge. Pro tempore have served as judges in the past during impeachment trials. Constitution allows the Senate to make the rules regarding the trial. Only stipulation is the judge if impeachment involves a sitting President.