Just saw the movie on TV last week and it was the climactic battle scene. The Enterprise is trying to rendezvous with a Federation fleet, but are intercepted by Shinzon in the "Mutara Nebula" analog, supposedly fairly close to to the location of the Fed fleet....who is aware of the situation. So the Enterprise is obviously late to the meeting, and the Federation does not bother to send any ships to investigate or find the Enterprise.
I doubt if they had crossed to the Federation side of the neutral zone so the weren't going to break the treaty first.
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!).