The right has plenty of ammo for Romney. He lost a presidential election (the irony of voting for one guy who lost a presidential election over another guy who lost a presidential election would be 100% lost on them).
But he could capture a lot of Republicans who hate trump and also still retain democrat voters. It would drum up a lot of press and could be viewed as something new unified front against the Republicans who have turned into a party devoted to a single guy
I'd rather not have to worry about the possibility of MR becoming president, should KH die or something (I don't think she will, but there's a non-zero chance anyone could die).
MK would probably make some popular choices but try to not rock the boat much. That would be more tolerable than President Romney.