What exactly was the point of the scene where they find B4 and are chased? What would have happened to them had they been caught or killed? What was Shinzon's plan with that?
Shinzon's plan was to wait until he was a week from death, then lure Picard with the lure of exciting offroad action so he could capture Picard. Shinzon likes playing the odds, you see.
It's unclear what would have happened if 1) the Enterprise failed to notice the positron emission, 2) the Enterprise did the prudent thing and took several weeks to return B-4 to a Starfleet research facility, 3) Starfleet sent regular ambassadors to meet Shinzon instead of Picard, or 4) Picard was killed while playing dune buggy.
I doubt there was a backup plan for any of these things. I suspect the Shinzon would have hit the fan.