According to the books, Tal'Aura (the one who smuggled the Thalaron-Generator to perish the former senate) immediately took leadership and received support by most military, while Donatra (the one who joined Picard during the Battle of the Bassen Rift) called for a revolt and eventually lead a seccesionist party that formed a separate Romulan State. I find this nonsense. Even if we agree that Tal'Aura had most of the military on her side to do this stunt, we never saw her afterwards, so either she 'suddenly fell from the Emperors grace' and got stationed elsewhere, or Shinzon disposed of her (latter is more likely as she's a conspirator and thus better off dead and unable to speak uneasy truths).
I think that sounds bad, too. It makes more sense for Shinzon getting the throne being the plan all along. That way Shinzon doesn't have to be magically handwaved into power, and the explanation can be as simple as the military gaining plausible deniability. "Who would put a slave on the throne!?" "As servants of the Empire we must stay out of this purely political issue!" All along, their plan could be to manipulate Shinzon from behind the scenes. I can even see that working, because Shinzon's hate was toward humanity and not Romulans. He was so focused on Earth and Picard, that he probably did everything the military wanted, while asking relatively little for himself.
True. But Romulans and honourable are to opposing terms.
Despite what seems like a back stabbing nature, I believe this is really how they treat enemies, and not each other. I like how in TOS the Romulans seemed driven mostly be duty, and its related honor, and I believe some of that survives into TNG. While any Romulan may be looking for leverage, I think they have to honor agreements, even if only to the letter. That would mean, if Shinzon agreed to help, and made freeing the Remans part of that deal, then the military would, at worst, not fight against that decision. That's exactly what I would have them do, if the military openly took over. I know it's not great reasoning, but I think it at least makes the Romulans interesting if that reasoning were used.
Besides, seeing the backflashes how the Romulan guards were treating the REmans, it's highly unlikely that the Romulan population would accept Reman leadership or even remotely Reman equality.
I completely agree, it's why I think the explanation for them having growing political influence has to be flawless. If the explanation amounts to "a wizard did it", I'll be severely disappointed.
Then they really build ships extremely fast...
Yeah.

I realized it after typing, that even one year would be too little time for a ship that big. I used to think that maybe the military had been building the ship for him the whole time, but I think it's more reasonable if it were being built regardless of Shinzon as part of the thalaron project. That way, Shinzon can take the project before interiors are installed, and have it fitted for Reman comfort.
[Remans] build a fleet with the Scimitar as its flag ship.
I don't see how this is remotely possible, for the Remans to have built and manned a fleet capable of taking on the Romulan fleet, let alone have a large enough army to fight a Romulan army. It's multiple worlds versus a single one. Even if much of the fleet has to stayed tied up with peripheral affairs, there should still be more than enough ships to put down a Reman uprising.
If there were a Reman fleet in the movie, the Romulan fleet shouldnt' have existed anymore, not as an independent body, and Shinzon should have had escorts, just in case.