Let Me Be Blunt...

Mitt Romney IS NOT running for President in 2016 on a "Save the GOP" third partei ticket. He hasn't the guts and something like that locks him out of any future consideration in the Republican Party Going Forward.
The rumors of "discussions" with William Kristol etc is likely nothing more than chin music with scared and aggrieved parties nothing. The Romneys, Bushes, Ryans, McConnells etc of the GOP lack the intestinal fortitude for such a move. They will likely simply withold their support for the Trump Ticket (Here, The Donald has made it easy on all of them by personally insulting almost every notable personality in the GOP in the past year) and bleed their hearts dry for the poor unfortunates that are running down ticket.
Indeed what else can any of them do?
The game plan now comes down to making sure Trump runs up an awesome smash this Fall, pose and posture as the friend of the endangered GOP down ticketers then rebuild after the dust settles. Its what worked for Richard Nixon in 1964 and seems the only play left to any of them.
Keep in mind the primary goal of this lot is retention of control over both the money supply and the ideological tone of the National GOP (they don't however as of this year seem to control the actual nomination however). They've take any setback this November as long as they can set terms over the party's revival.
They key problem here however is simple, in no ways does it recognize a historic shift within the GOP Base, the white working class southerners that deserted George Wallace for Richard Nixon and then marched for Ronald Reagan have been bought off every four years for short money when it came time to assemble a Presidential Coalition.  Now they are demanding more and things at variance with Movement Conservative Orthodoxy. Its a belligerent cohort and Trump has made them his own....Ryan-Romney et al, don't seem to understand this at all.
On the other hand if the same old same old stays in power in the GOP I'm not sure where that cohort goes, they can't backtrack to the democrats and the GOP has openly proclaimed its intentions to ignore all their issues.  This is prime breeding ground for another Trump going forward assuming the original does indeed go to smash and HRC's 13 point lead gives me scant comfort even now.