You will clear US customs in YYZ and then enter the US pre-clearance area at YYZ. The area is separated from the rest of the gates at YYZ which is why you cannot get to the KL lounge. DL uses the KE lounges (not KL) at ICN and you will have access due to your Z fare. You won't have access to the KE First class lounge (reserved for KE FC customers), but there are two other KE lounges where you will have access to at ICN.