I'm thinking they may not actually name an established republican to run independent, that's too blatant and would just throw even more support behind trump as they're viewed as openly manipulating the process.
The entire point is to pick an established republican. They need a sacrificial candidate for public positioning and branding. If Mark Kirk doesn't publicly come out against Donald Trump, the GOP has no hope of keeping the Illinois senate seat. If Rob Portman doesn't publicly come out against Donald Trump, you can say goodbye to Ohio. And so forth, Pennsylvania, New Hampshire, Wisconsin. That's just the baseline. Imagine how much fun it would be to try and run for the open Senate seat in Florida if everyone thinks you are a Trump supporter.
So then the republican candidates are put in a position where they get a bunch of question along the lines of "how can you say you oppose Trump but you are staying neutral in this race?" They can't go endorsing Gary Johnson or Virgil Goode because:
1) They're stuck endorsing an agenda that makes Bernie Sanders look like an establishment sellout
2) It looks like the party is falling apart if huge number of republicans are endorsing something that isn't the republican brand
3) It doesn't look like a sincere endorsement (because it isn't)
So in order to protect their brand they get someone like Mitt Romney who is:
1) A plausible candidate
2) Not going to saddle them with baggage
That way Rob Portman can endorse Mitt Romney or whoever and lessen the Trump baggage. When reporters say "why aren't you condemning Trump" he can say "I condemn Trump for being bad and for not being conservative and that's why I endorsed Mitt Romney".
They aren't picking a candidate who they think they can win. They're picking a candidate to make losing hurt them less in other 2016 elections.