I have a theory about that. The Romulan Commander
did not fall for Spock. I first posted it
here, and I'll restate it now:
The way I make this episode work is to say that the female Commander and Sub-Commander Tal were
both in on the plot to steal the cloaking device. They're like the American spies who gave the A-bomb to the Soviet Union. This whole plan was their idea, and they communicated with Starfleet so carefully that Kirk and Spock don't even know who the friendly Romulans are. They just know their mission is likely to succeed because somebody on the inside has it wired.
Thus the Commander and Tal do a lot of play-acting, including being upset when the device is missing, and the Commander shouting "Destroy this vessel!" when the Enterprise is about to escape. She knows Tal won't really do that. He'll make a show of trying, though.
And she grabbed Spock to escape with him because otherwise she will be put to death soon, whether or not the Romulan High Command figures out she's a traitor. Her feigned incompetence alone, that allowed Kirk to succeed, was great enough to get her executed.
So while Spock thinks he's "playing her" with a romance scam, she's just
playing along very helpfully because she can't risk telling anyone that she is committing treason. It could be overheard, the way Commander Kor's office is nanny cammed in "Errand of Mercy." She knew what Spock's game was, helped him play it, and let him think he was masterfully manipulating her. When the time is right, she will defect to the Federation rather than be traded back to the Romulan Empire in a prisoner exchange.
Did the writers know this? Of course not. They were making '60s television. But I think this "no-conflict retcon" holds together pretty well, and it cures the otherwise futile and suicidal nature of Kirk's mission.