You can absolutely use DL miles to get to SYD on KE - my fiance and I are taking our honeymoon in April flying open jaw IAD-ICN-SYD and then ACK-ICN-IAD.

It cost 150K DL miles each for Business class.

Obviously it was not ideal to either have to go through ICN or to have to sue so many miles but we really wanted to go to Down Under, I REFUSED to fly coach that far and, at the time we were looking last fall anyway, the cheapest business class fare I could find was $8K each. DL was the only airline that I had a ton of miles saved up on, so it was our only option that met all the criteria. And I would rather add on the extra flying hours in Business than have the shorter flight in Coach.