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.
While Pence would be the "cleanest" choice, in terms of convention credibility and Trump's support, I just don't think he can win. And if you are Ryan, Priebus & Co. engineering all this, you want at least a semi realistic chance of winning. Why go through all the trouble only to still lose - they might as well just leave Trump to do that.
I agree that Romney, who would otherwise be a good candidate, has ticked off too many people (and I don't mean just hardcore Trump supporters), so the only viable winning options are Kasich or Ryan. Mind you, even if they are chosen, they face a very uphill battle at this late stage. But if there is a candidate who may be vulnerable, it is Clinton. In spite of his antics, Trump has shown that clearly.