I rewatched Nemesis for the umpteenth time last night (an "umpteen" is defined as ten to the many) and, for some reason, I never noticed before: they make a big deal out of defining thaleron radiation as something that only destroys organic matter. In other words, it would destroy people, but not machines ... more to the point, not androids. So, that wouldn't have killed Data. What would have done him in would be the explosion of the Scimitar, except that you clearly see large chunks of debris being thrown off. So, Data's head could easily be floating around in that nebula or rift or whatever it was. B-4 has a perfect body but a malfunctioning brain, Data's head could be retrieved and attached to B-4's body ... and there you go. I don't know why I never noticed any of that in previous viewings of the movie.