Nuts.
All genuine Republicans (the non-hacks) need to settle on Hillary. She is really the only viable option at this point. Johnson has zero chance, and a vote for his is essentially a vote for Trump. I don't usually buy into that way of thinking (I voted Johnson in 2012), but your choice is between fascism or more of the same. You may not like the way things are right now; you'll hate it even more under President Trump.
If republicans are voting for Johnson it's taking votes away from trump not Hillary
True and looking at 2 way races, Clinton does better than in 3 and 4 way races. I think most of the Republicans not voting for Trump are married women who find him appalling. Let's say in theory moderates and Independents haven't come to his side, then I'd estimate up to 85% of Republicans supporting him at this time. I haven't seen GOP numbers lately though. Trump started out at 2% when he entered. By the primaries he had 33% of the party's support. When the primaries were over he'd gotten just under 50% of them to vote for him and 61% said they'd vote for him in the general election. Around the time of the conventions he had 66% of GOP voters supporting him. If Republicans gelling around him is going on, then I'd say it's roughly 85% based on recent polls.
I saw a figure showing 93% of Republicans backing Trump, but I don't have a source, sorry.
That's self-identified Republicans though. Likely a lot of Republicans not voting Trump aren't identifying as Republicans right now.