I believe your math is wrong on the first group, both that is bigger than Collins, and that she has any chance of winning by voting for his removal.
The reality is, I don’t think any republican up for re-election, can win by voting for Trump’s removal. They math is pretty simple, which has them between a rock and a hard place…they can’t win with just Trump supporters and they can’t win without Trump supporters. The caveat to that statement however is, they certainly cannot win if Trump turns the entire election into a grievance campaign about how badly he is being treated, which he certainly would do.
The problem with your second grouping, is people like Romney and Murkowski, are not running for office in 2020, so they do have a bit more leeway in their vote than people like Gardner, Collins, McSally, etc.
They have a Senate Majority to gain. And that ain’t nothing. Particularly when eyeing a Democratic President moving into the WH in January 2021.
IMO, its actually going to come down to republicans running for re-election in 2020, and it IS going to be difficult to convince them to vote to remove. Because of the above math. If you are Collins, McSally, Gardner, Ernst, Perdue…and a few others, you want Trump to resign and avoid the trial altogether.
There is of course the possibility that gets brought up that the Senate could change the rules and make it a secret ballot. I find that an extremely unlikely scenario. McConnell is crafty enough to see that there isn’t a good outcome for him or his party if that happened…if Trump IS removed that way, EVERY republican will be accused of voting against him by the Trump supporters (and McConnell himself would be hung out to dry for making the change). If pressured into considering such a change, look for McConnell to poison the vote by inserting other changes that will never pass, thus insuring the whole thing fails.