I think it was stated in Nemesis that B4 would never fully integrate Data's datatransfer, his technology simply was to...simple, compared to Data.
As for the half humanoid/half positronic brain, that was Vedek Bareil. He died a few days later....
An obstacle VERY easy to overcome by two whole sentences of technobabble.
It happened at least twice in tie-in fiction; both times, it was underwhelming (despite the technobabble being convincing by trek standards).
If that's your argument for Data's resurrection not coming to pass...well, they say 'hope springs forever'. Of course, 'grasping at straws' is more appropriate as a saying.
Well ofcourse they could change anything with a few lines of whatnot. I still feel that David Mack won't resurrect Data. Sure, it can still happen, and if it does, it does. Personally, I'm a bit 'meh' about the entire Data-subject by now. I prefer for him to stay dead, since death is death. But Trek has shown to overcome that little obstacle when it serves... well what really? Fan pressure? Plotlines? I don't know. All I know is, I used to be quite against bringing Data back. And I still doubt they will. But if they do, and write it well, I'm sort of ok with it.
David has never really disappointed me sofar (only ZSG was something I didn't really like by him), so if he does decide to bring Data back, I trust him to make it worthwhile.