Good news. Roberts can’t claim the right to break a tie in favor of witnesses, citing the Andrew Johnson trial.
Actually Article 1, Section 3, Clause 4 of the US Constitution would rule out Roberts as being the tiebreaker - it clearly states that the tiebreaking vote goes to the US Vice President:
“The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided.”