If they're trading for Navarro they'd be in contention so they wouldn't trade Iwakuma for him.
Navarro + framing this year has been worth -0.2 WAR.
Zunino + framing this year has been worth +0.2 WAR.
Zunino has been better this year, is cheaper this year, has 3 years of control left, and has upside with the bat from his MiLB numbers.He's just a better asset and the Mariners have no need for Navarro.