Romney will step down the first sign of trouble and use that as a starting off point to Primary Trump
I can easily envision Mitt publically appearing to follow Trumps wishes as SoS, while quietly undermining Trumps foreign policy agenda in his meetings with foreign leaders. When confronted by Trump, he will lie and say negotiations fell apart.
Miit is a snake. Trump knows that Mitt is a snake. If Trump wants to throw a bone to GOPe, then choose a less backstabbing option, and not for possibly THE most important cabinet position.