I'm not sure even Kasich could beat Clinton in such a scenario. The GOP would be effectively tarred as "the party that just nominated Donald Trump", as opposed to him having won the nomination in his own right. I agree any of the other Republican candidates would be unlikely for similar reasons.
The only logical choice would probably be Pence. Sure he may have issues all of his own, but when compared to Donald Trump, he is already emerging as a bastion of sanity. The nomination in that regard would be one last mess to clean up. If Trump were to drop out it would likely be in a spectacularly messy fashion that would make it impossible for pretty much any Republican to win-- so it's about losing with dignity. If he does a good job maybe he can challenge Donnelly in 2018.
Pence is the only candidate the entire GOP could agree on.
I think Romney blew it by refusing to endorse the ticket, or at least indicate he'd vote Republican in November.