1. Someone should have controlled Spiner's influence on the script.
The bigger problem was Stewart, who got his Associate Producer title after First Contact made bank, and then he just got completely schizophrenic about Picard's character -- he wanted it to be a philosophical story, then when he got that he decided that Picard couldn't be weak so he needed to be an action hero, then when he got that he felt that he needed to have a romantic plot, and when he got the romance subplot he said it was just too much.
Let the actors act and let the writers write (Spiner having a favored-nation contract meant that he got to impact the story of Nemesis).
For those of you who haven't read it yet, what Timby says here is pretty much the crux of what Michael Piller's book, FADE IN, actually says about the writing process behind the movie.

I tend to think of INS as being Stewart's one "primma-donna" moment on Star Trek. He was (and is) a very giving actor, but that was the movie where, flush from the success of First Contact, he really weilded power. So he finally got all of those things he said he'd wanted for seven years of the TV show. What Stewart couldn't see was that his suggestions were unbalancing the movie.
Spiner did give notes on INS too, but most of those were pointing out the logic problems of the story (and not, in fact, related to making Data more central to the plot -- that was one of Stewart's ideas). In fact Piller goes as far as to suggest that Spiner only gave sound advice and that he wishes he'd listened to it and ammended the story accordingly.
The studio heads also gave notes about the logic problems, but again Berman/Piller opted not to take those notes on board. So we got the movie we got despite at least two seperate parties pointing out significant problems with the script.