There's no denying that watching the Republican candidates self-destruct has some entertainment value. Also, it is clearly paying the rent for a lot of pundits. What I haven't seen is any leaks about what the heavyweights in the Republican establishment are really thinking in regard to a Plan-B. Romney's recent comments certainly don't qualify as they were clearly just spin with little thoughtful content.
Trump has already changed the landscape for Republicans in a way that may be irreversible. Allowing him to actually step over the White House threshold would be unthinkable for them, not to mention the rest of us. At the moment, that leaves them with Cruz, a man who seems able to inspire revulsion everywhere except in a small corner on the far right. So, having quite likely written off Rubio, that leaves the R's looking for someone who can make reasonable sounding campaign promises, and who won't rock the boat excessively if they actually get into office.
It seems to me that the obvious choice for the Republicans is Hillary Clinton. Listening to her answers in the Flint debate on fracking, on campaign finance reform and and on trade agreements, it is pretty clear that her plan is to chip away modestly at the status quo rather than to have a go at a serious revolution as Sanders proposes. If she does make it to the Presidency, she will likely not entrain enough congressional victories to recapture a majority in either house, meaning that the Republicans will retain the capacity to block any kind of reforms at all. And, of course, they will have the fun of casting the blame for government's failure on Hillary, just as they have done with Obama. Seems like a no-brainer.