In those private moments, when it's just GOP senators by themselves, are they discussing the possibility that if they vote en masse for impeachment, they can reduce the blowback that Trump can direct toward them as individuals?

Yes, but I believe these discussions are taking place on a bipartisan basis basically. They are all talking to each other about this mayhem. But sure, absolutely when high ranking Republicans sit down to discuss this they most certainly are weighing cost and benefit to the party. 

Will it come down to that, Reg? 

I think if you want to look for signs of what is happening first ignore the group of clowns that come out and talk to the media to "defend" Trump. So, ignore Graham, Rubio, and of course all the total liars like Jim Jordan, Mark Meadows, Devin Nunes, and Mitch McConnell. Whatever these guys say or don't say is generally only meant to throw mud in the water. Not important. Plus Graham changes what he is backing or not backing from moment to moment. Right now he appears not to be backing Trump but who cares because tomorrow he could be back up Trump's ass. 

I would instead watch a guy like Richard Burr because what he says, and he is going to see all the info on this, could swing a lot of Republicans. I don't think the Republican senators are hanging together, they are no longer a wall...or if we see them as a wall it is a wall being shaken by an earthquake that is wobbling and crumbling by the day. Don't expect it to all come down at once but as more and more comes out more bricks will fall.  

I think the Republicans and Trump were trying to get the Democrats to speed up the impeachment process for the specific purpose of having less come out. Every day there is some new damning information or testimony now. Hell, I just saw recent video of Lindsey Graham and he looked shaken to his core...and was not at all behind Trump in that moment.