But I'd argue if that Durkon never entered the order then the prophecy wouldn't exist, or some entirely different prophecy would have been divined.
Also I get the irony of the Croesus story, however ... I'd argue that even though different outcomes could satisfy the literal interpretation of the prophecy, there's still only one outcome that was going to happen. e.g. it's highly implausible that there was a coin-flip which neatly resolved into exactly two possible outcomes, Lydia destroyed or Persia destroyed. That is far more unlikely than the simple determinism scenario, where he was given the prophecy, which determined his course, which determined that Lydia would be destroyed, thus informing the original prophecy.
The idea of non-deterministic prophecies would seem to only apply to prophecies which are deliberately vague, e.g. contrived cases.
Back to the point however, there's no special "magic" needed to explain how prophecies come to pass. e.g. it's not like you have a prophecy you're going to be eaten by a lion on one path, and you have a choice between left or right, and get eaten by the lion "either way". Because the prophecy already takes your choice of right or left path into account. e.g. if the lion is on the right path, and you choose to go left, then the prophecy wouldn't have been made in the first place. The prophecy itself only exists in the universe where you decided to walk on the right hand path, thus getting eaten by the lion.