After the Enterprise is given official sanction to cross into Romulan space and it is being pursued by its Praetor ? Safe to say the time for the olive branch was long over with by that point. Considering Picard was in starship combat with the leader of the Romulan Star Empire ,Starfleet would have nothing to lose by sending backup.Shinzon can't file a diplomatic complaint from the grave or a Federation jail cell. Nemesis failed in the first duty of a movie, and that is to entertain. There are plenty of implausible movies which are fun to watch nonetheless. Nemesis is implausible, illogical, and depressing. At least in Insurrection the characters had some positive experiences.( if only the Geordi sunset scene could have been the whole movie!).