If you ARE taking Nemesis onboard, then you can't get away from the fact that they practically bloody signposted "THIS IS HOW DATA COMES BACK!!!" in it. No half-measures.
Actually that's a misreading of the filmmakers' intent. If anything, the story makes it extremely clear that B-4 cannot
become like Data -- the attempt to download Data's memories to enable him to grow and improve is a complete failure, and Data comes to the conclusion that B-4 is simply too limited and incapable of growth, which is why he ultimately shuts B-4 down. That's actually a crucial story point, because it thematically parallels Shinzon's inability to grow and change, which is the whole thing that makes Picard (and Data) better than Shinzon (and B-4). The whole thematic unity of the movie depends on B-4 being incapable of becoming Data. The bit at the end with B-4 remembering the song is just meant to give a slight note of hope at the end, a sense that maybe Data's legacy endures in some small way.
Yes, the studio no doubt insisted that a "back door" for Data's return be included just in case the movie was so hugely successful that Brent Spiner could be talked out of his decision to retire from the role -- but that was just a contingency plan. Too many fans mistake such contingency plans for the explicit intention of the creators, but they're not. They're just a way that filmmakers hedge their bets against the unpredictable. In this case, the filmmakers' intent was clearly for Data to stay dead permanently; after all, the whole reason Spiner came up with the story is because he thought he was getting too old to play the role anymore, and he wanted to bring that part of his career to a decisive end. But the studio insisted they hedge their bets ever so slightly -- and ever since, people have been mistaking that hedge for their primary plan, which is missing the whole thematic point of B-4's presence in the story.