Not to equate the two, but Lincoln’s arch-rival was William H. Seward, who said unkind things about him when they were both competing for the 1860 Republican nomination. Still, he made him his Secretary of State. At first Seward continued to consider himself Lincoln’s superior. But Seward soon learned who the superior was, and he actually turned out to be a very supportive and useful SOS.
Lincoln also was able to figure out what to not take seriously from Seward too.
Romney did all those things. I agree, he should not be in the cabinet, and I believe he will not be. Just talking to him and flattering him with the possibility will keep him in line for now, and help bring the nevertrumpers back into the fold.
Trump also spoke with Rahm Ehmanuel's brother and with Tusli Gabbard, and I think he was doing the same thing with them he was doing with Mitt--schmoozing his enemies and asking for their advice while not really trusting them.