If it's Yeb, so be it. No difference between Yeb and Hillary anyway.
I have no use for Trump and think he's a political fraud, but if he runs Third Party and acts as spoiler for Jeb, I really couldn't care less, just as I really didn't care whether we got Clinton or Bush I/Dole in 1992/6.
The RNC has overplayed its hand of "vote for whoever we pick for you, because the alternative is a liberal Democrat." The reason this no longer works is that the RNC pushes candidates who could just as well run as Democrats.
If the RNC supported halfway decent candidates in the primaries for a change (something they haven't done since 1984), they wouldn't have to worry about third party challengers or low voter turnout to begin with.