I don't think republicans generally want Roberts to preside over this thing. He's the republican appointee turncoat that said Obamacare is constitutional. Lots of republicans, particularly the diehard fascists, hate Roberts with the same amount of venom they hate Clinton. It's as if Hillary will be presiding over proceedings in the senate. This is not something Senate republicans want to come about and is why they are fighting so hard to see that it does not happen. They don't control Roberts and they know it.
Trumpo and his lieutenants will target him next if it goes to the Senate so it's gotta go to the Senate if for no other reason.
Roberts is the least of their worries. It is the moderate Republicans (the two or three in the Senate) that can help provide a majority vote. It won't boot Trump from the White House, but the optics of the Democrat impeachment receiving a majority vote in the Senate isn't good. The GOP have enough issues trying to spin what is public record now, forget trying to spin losing a majority vote in the Senate while holding the majority of seats.
The Democrats need four votes in the Senate. Sen. Collins is doomed to losing in Maine, so she seems likely. Sen. Windsock in Utah definitely is a threat, but as we know, he is a windsock and changes his position every second of the day, in fact I bet he could vote 'present', if allowed. I'd say Sen. Doug Jones (D) is a dead man walking so he is free to vote appropriately. Sen. Cory Gardner is on the edge and likely facing a major uphill battle in Colorado, so he is up in the air. Sen. Tillis in North Carolina appears to be tacking to the right. Sen. McSally appears to be doing likewise, which will likely end her short Senate career. This leaves us with Sen. Ernst... who isn't looking at an uphill battle for re-election, but it isn't a cake walk either. So there are 5 or 6 votes that could potentially be up in the air.