No need for tie breaker, unless Roberts wants more witnesses.
50-50 is not a majority, so whatever is proposed (more witnesses) doe not have majority support.
Odds are that the senate will vote 49 for more witnesses. This is also not a majority.
I don’t think Roberts wants to spend any more time in the loonie-bin known as the senate, than he has to.
Well, Roberts is a known never-Trumper, but I think he can see that removal is not ever going to be in the cards, so he will just stay silent.
There is no good precedent and if he says "yes" to witnesses but doesn't allow Hunter and Ciarmella he will be seen as overly partisan, and will earn Trump's undying enmity.
Even one of the Rs who will vote for witnesses says she wants equal numbers of defense and prosecution witnesses. The Ds do not want Hunter under any circumstances, Joe is a wild card, but could be a disaster for them, and Ciarmella will raise many questions of Schitt and his staff.
In the back of my mind, I am seeing one D defection on witnesses to put the nail in the coffin...