Shinzon needed the military's backing. You can't kill the government and taked over. It just can't be done. You have to gain control of the military first. If Shinzon hadn't had a plan to destroy the Federation, he's have never recieved the Romulan admiralty's support, and they'd have blown Remus out of the sky right after the attempt.
What I hate is the ever-shifting cannon. Up until Nemesis, the two planets in the Romulus system were Romulus and Romii.
Nemesis sucked because the moral battle was difficult for us to understand, because we're so used to people blaming their problems on their surroundings. How many times have we said 'Oh, that poor child must have horrible parents' or 'Its not his fault, poor guy was under a lot of stress', or things like that? Plenty.
Picards a good man, though, and he's the kind of man who blames himself first. Its the mark of a classic hero that he won't flinch at his arm being sliced off, but that he'll break into tears if he gets a friend killed. Picard looked on that and figured that all of his personal moral intergrity just came down to his upbringing and situation; something most people already believe. Also, their solving of it wasn't that inspiring. Basically, it came down to 'So what?', which is kinda cheating.
Alexander