But I don't agree about including Sela, Tom Riker, or even Lore. I think there's probably too much backstory in there for a feature film. Lore maybe. You could have a line of dialog about how Lore is the prototype for Data that went horribly wrong. But Sela? There's no way to explain who she is, and why we care, in a line or two. And we don't need time in the movie devoted to explaining the backstory of Tasha Yar, "Yesterday's Enterprise," "Redemption," and "Unification".
Personally, I think the clone of Picard idea could have worked. But it should have been Patrick Stewart playing both characters. He is a good enough actor to pull it off without it being corny. And we didn't need a Data story where he also happens to be dealing with a clone. Not every movie needed a Data b-story. Really. Data could still play an important role, and even sacrifice himself in the end, without having to have his own storyline.
You're right about Sela, it would probably too complicated to explain her existence within the movie. I also agree that Patrick Stewart playing Shinzon might have saved the whole clone plot. But it would still be an odd story, that I would rather drop alltogether. I think that Lore would have been the better (and obvious) choice for the a-story, with Romulan politics as a b-story ... Shinzon as the new Romulan (not Reman) praetor, and not as Picard's clone.