How could they be sure that Soneji would kidnap Megan?

The entire plan of the kidnappers who use Soneji is Sonenj's plan. This Sonejy guy craves fame and he would like to perform another crime of the century like the one about Charles Lindbergh's infant. He hatches a plan to kidnap the daughter of a senator at a private school and two rogue secret service agents found it out and let him do it because the plan to kidnap the girl as well, demand random, kill the girl and blame Sonejy. The whole thing might work only if Soneji sticked to his plan. But what if something was wrong? What if Megan had successfully escaped as she almost did, and what if Soneji had killed her because he was tired of this fame game? The other kidnappers could not do anything about it. It is a very loose movie that is strecthed so much to defy basic logic.