You loved her. You did the very best you could with the information and advice you had. And in the end you chose ending her suffering over trying to force some kind of win/survival. I admit that in your situation I'd probably have kept going (and kept her suffering) out of sheer pig-headed...
Pulling this completely out of my butt, do you guys think there's any way @Opal could try to feed the yolk to the duckling orally? Maybe putting sugar on it so it's more tasty?