A little accuracy please.
Ken Starr, gave his opinion, other Constitutional scholars disagree with him
John Roberts is NOT Judge Roberts, he is Chief Justice Roberts. T are Justices, not judges. Please give him his due.
The reason Roberts will not preside (it is in the future, not past as you wrote) is because the Chief Justice ONLY preside at the Impeachment Trial of the President of the USA. Not the former President, not a judge, ambassador, postmaster or other federal official. The Senate gets to choose one of its members to preside at all impeachment trials except of the President. Trump is NOT President, thuis no Chief Justice presiding at his trial.