I'm not usually one to pick the movies apart. I try to see them for what they are, each given its own context, but ...
If Shinzon had been Picard's illegitimate son, the "if you had lived my life" crap might have made more sense. To me, "You abandoned us!" plays better than "I was supposed to be you!" Still a little Khan-ish, but it could work.
Substitute an incurable genetic disorder for the cloning malfunction or whatever that was and let it play into the rage and derangement. "YOU DID THIS TO ME!"
Lose the whole B4 nonsense. It had already been done to death with Lore.
A different B plot--any B plot--would have been better. How about preventing the Remans from taking over Romulus after killing off the Senate?