So this means (if we call it a ruling) that Cargo Ship with no set-aside card is discarded in clean-up. Donald says it's discarded even if a card might be gained later in the turn. To me this means that we only consider the current game state at that point (when discarding in clean-up) when evaluating whether the card has something left to do after this turn.
I think your conclusion is correct, but what I'm saying is that Donald's last post that you quote here, while giving a "ruling" that applies to Cargo Ship and Garrison, was also said in the context of only talking about whether Durations should be "have something left to do" or "will do something on a future turn". Although the quote mentions Cargo Ship, it could just as easily have been about Squirrel or Seal instead (and I think everything he said does apply equally to all of them). He doesn't say anything that's specific to Cargo Ship's unique situation (which is the same as Garrison's).
All I'm saying is that a rule specifying that Durations need to do something next turn, and not only "have something left to do", isn't sufficient. Another rule is also needed which addresses cards where you don't know if they will do something next turn.