You can still complete evacuations even without doing the elf village related quests. Just talk to the head elf every day for about 3 days and he'll give up and evacuate.

Now in regards to the elf girl, tried reloading an inn save?

Thanks! That's great to know about evacuating the elves without this quest. I'm sure reloading the inn rest would have worked, but when I realized I had a huge problem on my hands I had already rested at an inn again.

the oracle says "Seek audience with Sarah, who will grant you a task." Talking to Sarah, however, does not help, and she does not grant me a task.

I couldn't find her too, turns out she was waiting for me just next to the cauldron where she cooked the nutriabh. Isn't she there in your playthrough?