How is it even possible that Elsa never learns her name? Someone has to menton Evies name eventually.
At this point, we just have to suspend our disbelief for it. The author is committed to the bit, so we'll just have to accept that as the story progressed, it ended up that they spent some extended time together in the past, and whoops, that didn't go with Elsa not knowing Yvonne's name. Gotta write around that, but what can you do, it's just a problem of episodic writing.