From Julius Caesar to Romulus Augustulus to Thomas Palaiologos, you name me one Roman Emperor, co-Emperor or heir of either the whole empire, or of the Western/Eastern periods who were of a different species other than human.
Shinzon does not gain control of the Romulan Empire without the help of Romulans. You can do all the backwards somersaults you want to work around it. It simply doesn't happen.
The only thing backwards here is the retcon of Logan for his Remans and clones. The only reason it happens at all is due to a piss-poor story with weak and (supposedly) scary-looking bad guys + a melting clone of kid-Picard.
Riker even says in the movie that the praetors power has always been the Romulan fleet. Shinzon does not take control without the support of the fleet.
Riiight, the xenophobic Romulan military not only trusts slaves with guns, but also a clone
of a non-Romulan that exposes their plot to infiltrate the Federation. Riker can say whatever he wants in a movie full of silly mistakes and unbelievable situations. It doesn't change the fact the Romulan military and Tal Shiar would go apeshit at even a hint that a clone was planning to take over their Empire. The movie itself even proves this as a highly probable scenario by the fact that someone who backed this guy - DONATRA - decides that Shinzon isn't someone worth any level of trust or loyalty.
Wow, a Romulan
who can figure out that a clone of an alien who leads a slave race probably isn't a good candidate as leader of the Romulan
Empire. Crazy talk, man! Who'd of thought of that?
Early in the film, they're on the right track. They paint this image of Shinzon being this cunning, cold warrior who takes control of the Empire. But they present us a pimply nerd who is infatuated with Troi's hair and wants to have tea with the brother who had a better life than he did.
Shinzon rising to power isn't the problem, it's the fact that the character we see doesn't match the character we are told about.
His laughable rise to power is part
of the problem. Anyone who "get's" the Romulans and understands what makes them unique on a psychological level can see that. If you don't, that's understandable - not everyone delves into the inner workings of every Trek race.
In contrast, the Romulans have always been my favorite Trek race, mostly because I "get" why they are xenophobic and arrogant about their place in the universe. Not every Romulan is the same as the rest, but most have a certain behavior which they follow. In order to understand what makes them different, IMO, the individuals who define them best are the Commander from The Enterprise Incident
, Commander Teibok, Sub Commander Taris, Ambassador Nanclus, Centurian Bochra, Admiral Jerok, Commander Tomalak, Senator Pardek, Sub-Commander Selok/Ambassador T'Pel, Commander Sela, Mirok, Varel and Parem from The Next Phase
, Commander Sirol, Senator Letant, Nero, hell - even the damn soup lady in Unification.
But if you really want to understand their psychology and what goes on in their heads, I have a single episode for you to watch: In the Pale Moonlight
- DS9. Both the exchange between Dax and Sisko, followed by the meetings Sisko has with Senator Vreenak and the subsiquent "aftermath" of the meetings
demonstrate precisely why the Romulans in Nemesis are written both clumsily and inaccurately.