I understand, correct me if I’m wrong, Justice Kennedy can rule on the 9th Circus decision against President Trump, or he can pass the decision up to the full court. Justice Kennedy is like the gatekeeper for the 9th Circus.
Kennedy in no way, shape or form can unilaterally overturn the 9th Circuit decision if it rules against Trump. All Kennedy can do is (a) decide whether the full court hears the appeal; (b) continue or end the stay pending such appeal. On the former, if he denies the appeal the 9th Circuit decision stands. If he grants the appeal it goes to the full Supreme Court for further review and final decision.