It just seems to me that there was a purposeful setup at the end of Nemesis that Data would eventually reemerge through B4 (to me Nemesis was just an elaborate rewrite of TWOK).
It wasn't a purposeful set up, it was a safety net. Writers in an ongoing franchise can't make assumptions about what subsequent storytellers will do, or even whether there will be subsequent stories. Brent Spiner wanted John Logan to kill Data off in NEM, and that's what he did. But in the wacky world of movies, there was always the possibility that there could've been a sequel and enough money offered to Spiner to get him to come back. So they left a little bit of a loophole that could theoretically
be used to bring Data back if
things had worked out that way. It doesn't mean their intention was to absolutely, positively bring Data back the first chance they got, because filmmakers in an ongoing series know better than to make rigid plans like that. It just means they wanted to hedge their bets.
Consider your own TWOK analogy. The final shots of TWOK, with Spock's tube intact on Genesis and Nimoy giving the voiceover, weren't put in because the filmmakers knew for a fact that Nimoy would be returning, because at that point, they didn't know any such thing. They were put in because test audiences found the ending of the film too depressing, so a more hopeful grace note was added. B-4's little song at the end of NEM serves the same purpose -- to offer a glimmer of hope to soften the tragedy. It suggests that, even if Data can never come back from the dead, at least part of him lives on.
Maybe the situation is that Data is fully encompassed in B4’s subconscious, and it would take some extreme event for Data to emerge, though with some of B4’s limitations - making Data more “human”.
I just find it thoroughly unbelievable that a prototype as crude as B-4 would ever be capable of containing any sizeable percentage of Data's consciousness. Telling the story that way would require glossing over most of what was established about the character in NEM. At most, B-4 might be able to grow to the point of being a functional child with occasional flashes of Data's memory. And I think that's potentially a hell of a lot more interesting than resurrecting Data. We've spent 20 years exploring what it is to be an advanced android with superhuman intelligence and capabilities. What's it like to be an advanced android with a severe, crippling learning disability? There's a staggering irony in that. Also a splendid opportunity for social commentary. The story KRAD told in Articles of the Federation
about the legal debate over B-4's right to exist and be defined as sentient could never have been told if he were just Data 2.0.