Spock himself doesn't seem to have a high opinion on his opponent, or her species:
"You're being clever, Commander. That is unworthy of a Romulan."
Honestly, I don't see the gullibility angle here. The Romulans had to be careful and proceed in steps, because they didn't want Scotty or some other hothead blowing up the precious prize. OTOH, the Romulans were in no hurry whatsoever: for a rare once, they had not violated the RNZ themselves, and were safely on their own side, immune to Federation countermoves other than ones that would hurt the Feds more than the Romulans. Why not wine and dine?
...Because Spock
suggested wining and dining, doh! But Spock had already been "candid" about the tactical situation, spelling out his motivations and hers, and his logic on W&D sounded like a natural continuation of that. Sure, she'd be angry afterwards, like Kirk when going all "KHAAAAAAAN!!" despite actually being on top of the situation. And she'd have little reason not to let it show.
The silly business with inept guards could be more a function of those not being guards at all, but assorted nerds charged with operating the new supertech. The guarding would take place elsewhere, farther out. Note that nobody attempts to call a guard next to the cloaking device when the plot is uncovered - instead, a posse walks to the spot, as if such a guard were not expected to exist.
Why don't the Romulans raise shields? We know from "A Taste of Armageddon" that shields do zip in preventing a beam-out or a beam-down, even though they block a beam-in or a beam-up. But that's with a transporter that sits inside the shields. Kirk and Spock are extracted using an external transporter. Any clever ideas on that?
Timo Saloniemi