You guys have got to be kidding me. NEM is not about the Romulans. I just rewatched the film and counted how much time they are physically on screen. 8 minutes and 2 seconds. A movie that has an hour and 56 minute run time ( one hour and 49 minutes if you don't include the credits), and we a debating how out of character a race of people who were minor characters to the whole story of NEM.
Shinzon's Romulan collaborators you can count on one hand. 5 Romulans helped him overthrow the senate. While it was Commander Suran had secured the loyalty of the Romulan Fleet (which was off planet during NEM), Suran was the mediator to the fleet and Shinzon. Shinzon propped himself up on the throne and declared himself Praetor, it doesn't mean the Romulans had pledged fealty to him. In fact assassination and betrayal is common thing on Romulus.
I don't understand this whole thing with the Romulans being "so out of character". Dontra rallying Suran to help fight Shinzon, I interpret as the Romulans whole twisted sense of logic. While not the most honorable race, they aren't monstrous enough to exterminate an entire race of people. They are conquers. They are not an evil race. In NEM i found nothing so egregious by their portrayal to warrant such hate. I surmise there is some expectation that all Romulans have to be cold, calculating, and super intelligent villains who are always hiding their true motives. When in reality they are on par with humans in their development and technology. Cultures are different but as TNG The Chase showed, deep down they are similar to humans. The novel of NEM conveyed a sense that while the Federation and Romulans have been at odds, they don't instinctively hate each other. There is a scene in the book where Romulan medical teams help Doctor Crusher with the wounded on board the Enterprise. A female Romulan doctor saves Worf's life after he had suffered terrible disruptor burns to the chest.
The claims of xenophobia are bs also. Maybe you all have forgotten TNG Birthright where a Romulans had willingly lived with and started families with Klingons captured from camp Khitomer, or TNG Face of The Enemy where a human who had lived on Romulus for 20 years and had no trouble from the Romulan government or the Tal Shiar for being human. Shinzon was a proven warrior and had a plan to topple the Federation. He found Romulans who was support his plan and set out to execute it. Although he didn't seem to be in much of a hurry, and you'll note the dissatisfaction of Commander Suran at Shinzon's delay in action.
The hope for Trek in the end one day all races should be united. While NEM is more of an action peace, these ideals were in John Logan's screenplay. Due to focus of the director and length of time these were cut.