In "Unification", it provided a perfect excuse and perfect means for rounding up and eliminating forces of dissent. A popular movement that gathers various dissenters and is quite obviously treasonous is a godsend to a government wanting to control its masses, because everybody partaking can be freely executed without any destabilizing effect - the rest of the masses will cheer.
I'll accept that a portion of the masses will cheer, but everyone? The problem with Trek
is that we're never clearly shown what the average Romulan citizen thinks of its government. I think it's reasonable to assume that younger generations would see things differently than their elders (this seems to be the case in every civilization), but I have no idea who trusts the government and who doesn't.
The position of officers like Toreth is clear, but her feelings are based largely on her father's death or imprisonment by the Tal Shair ("I never saw him again."). Would she feel that way if her father were still in her life?
Jarok clearly had issues with how the military was addressing matters both domestic and foreign, but what his ultimate motivation for such feelings (beyond wanting peace for his daughter) isn't clear. Was he simply tired of conflict after so many years in the military? Did something happen under his command that made him doubt his place? Or was he going to through something of a mid-life crisis (as Kirk was in TWOK) but didn't have friends or a support system to keep him centered?
I've talked about Koval a great deal (he's probably my favorite Romulan character aside from Donatra). He's clearly a patriot and wants whatever benefits his people the most, but is he truly a man with righteous motives (king and country before all else), or is he merely an arrogant and misguided individual who believes he's right only because he doesn't have enough sense to listen to anyone else? We don't see enough of his character to know for sure.
And in fact, when Shinzon sat in that hall, it was already a case of the military establishing its latest puppet - at least from the point of view of the military!
I agree. It actually explains quite well why Donatra and Suran were willing to back him. If they saw him as a way to accomplish what they couldn't by themselves, it's absolutely possible that they'd welcome the removal of the existing Senate in order to clear a path for him. Once they realized that his actions would likely doom Romulus, they elected to stop him from attacking the Federation. That Donatra was able to broker a tentative peace (or at least a truce) with Picard was an added bonus.