I think Shinzon should have been Picard's son.
Then this movie would have been a validation of what Picard did and said in GEN and a reason for Picard to try to save him. By making it his son its no less a reflection on Picard. But he has more motive to save a son than an evil clone.
I would have cut Shinzon's creepiness. I just don't think he needed to be totally evil. He could have just captured Picard to take non-death threatening blood transfusions from him. Maybe it would have been better for him to try to destroy Romulus and maybe Picard would seem nobler if he was trying to save the billions on a planet he doesn't really like. Maybe he would sacrifice his only son (and Data to do it).
I disagree. I think it works as a clone. Picard has a bit of hubris in this movie--that Jean-Luc Picard would never do the things Shinzon of Remus has done. And, because he's a good man at heart, Picard can appeal to his higher sense of self to turn him into a good man. That is interesting for Picard's motivations as well as Shinzon's. It's more personal than when he tries to save other life forms. This is about proving that Jean-Luc Picard is a good man.
Making him a son makes it less important because he hasn't been in the child's life. It means that his hand-wringing at the beginning of the movie about how everyone is leaving him and he made decisions that left him without a family, are destroyed. He's inherently a bad father, and therefore, directly responsible for Shinzon's actions. That's a completely different movie that paints Picard in a very bad light.
Besides, how would you get DNA from Picard? Did a Romulan, um...well, you see my point. Did he know about this son? Why would the Romulans throw that son away when we've had Tasha Yar's daughter rise through the ranks? What is the motivation of the Romulans to get a son of Picard?
I agree with some of the points about the portrayal of Shinzon. Could he have just as easily turned the Scimitar on Romulus? Absolutely. That is a thought that I have had a million times.
However, he wouldn't be Praetor too long without the support of the Romulan fleet, as he eventually finds out when the Romulans abandon him. I think, like with most Star Trek movies, there is room for improvement. I think it would be a better movie if the Scimitar went up against 3 or 4 squadrons of Romulan ships instead of 1 The thing is the freaking Death Star. It could handle multiple attackers and still survive.
It's not perfect, but I see what they are doing. I wish people would stop looking for things that are wrong with it, and put their biases aside, and try and see what the movie is trying to do. The dialogue is b-rate and blunt, the acting performances are over-the-top, especially Patrick Stewart, (with too many catch-phrases) and it's needlessly plotting in terms of pace. But those don't speak to the question that was originally asked. "Could Shinzon's motives been clearer and would that make a better movie?"
I think it would've tied the movie nicely together if they had a flashback to Picard's weakest moment, most like Shinzon, with Tom Hardy playing Picard, so when the audience sees Shinzon, they know who they are seeing.