And if so, they would likely either stay home or vote 3rd party

...Look we can argue all day long about how justified perceptions of Hillary are and how much is the media's fault, or whoever. But whether we want to admit it or not she is not a very popular person and not a very popular candidate at all. If the GOP candidate were anyone but Trump she would, I believe lose fairly handily. Some of this is because of her unpopularity but some of it is also just the nature of elections, and she is essentially running as the incumbent in an election cycle where the incumbent typically loses.

The idea of Pence as the nominee would turn out Envangelicals in droves. They are already voting for Trump but having Pence at the top would solidify it.

All this being said, I believe it is almost impossible for Republicans at this point to change the ballots themselves. No matter what, in most places Donald Trump will be on the ballot as the Republican presidential nominee. Even if he drops out and all people in any position of electoral power in the GOP say "O.K. so if you vote for Trump, Pence will be the president that still causes a great deal of confusion. So I ultimately think they will still lose. But it could definitely put a tricky spin on it and maybe even make the race tighter.

It's almost impossible for anyone to say or predict, just as this entire election cycle has been impossible to predict or actually believe it could be happening.

If the Republicans ran someone like John Huntsman, Clinton wouldn't stand a chance. But they nominated a total fool and despite Hillary's lack of popularity and short comings she looks awesome juxtaposed to the orange idiot.

But if they nominated him, he'd have a better shot than Trump (almost anyone would). But I think he'd still lose. He is not that popular nationally, or even in Indiana.

It depends on how the electors vote

Be it Pence or another, they might look better having not gone through a year of tough campaigning, debates, etc.

The real problem would be how GOPers would get voters to still check Trump or Pence's name on ballot, or get them to write in if that were the approach, etc.