The consensus seems to be that Roberts will not intervene if it is a 50-50 split.
Which is impossible, whatever choice he makes he will be intervening.
He's the deciding vote in impeachment trials as the Vice President is automatically recused.
If it's 50/50 and he "declines" then it remains at an impasse.
If his position is that he "declines to interfere with the Senate Majority Leader's plans" for the trial then he is casting his vote in favour of no witnesses.
If his position is that having witnesses and evidence is the usual protocol for a trial and he's "declining to break from that process" then he's casting his vote in favour of witnesses.
I think what the consensus is around him "intervening" is that if the Impeachment Managers stepped outside the voting protocol and asked Roberts personally to subpoena witnesses and evidence for the trial then he is unlikely to intervene and I think there's a good chance that we will get to find this out because they are likely to vote evidence and witnesses down at which point the House Managers have absolutely nothing to lose by asking him to maintain respect in the courts and constitution and hold a legitimate trial.
I think the key thing which may help the Democrats is that McConnell is holding a vote to do something outside of precedent. The automatic position for any trial is for witnesses and evidence to be submitted, so if it's a split vote then it would make the most sense for Roberts to say that there wasn't enough votes for witnesses and evidence to be dismissed and therefore the trial must go ahead in the same way as every other trial in the US.