I'm with you that with the storylines, Zayn would be the best payoff to dethrone Reigns. But as you pointed out, it's probably going to be Zayn/Owens vs. Usos & Rhodes vs. reigns at Mania, which works too, but just not as good if it were Zayn.
Having said that, would it be best for them to keep Zayn out of the Rumble itself? I'm not expecting a 14-15 DB level hijacking here, but with the crown being invested with his journey for this long, is there the possibility that once Zayn gets eliminated, the crowd could turn on a Rhodes win because not only has he been absent for a while, but he also is the company fed obvious choice, like Batista & reigns were those years?
Put the heat on Reigns. Have Sami come in at #1 or 2 and soon into the run, Roman or Heyman comes down and demands that he prove his loyalty by eliminating himself, and then when he refuses, Roman does it himself.