Yeah, you see when you brought up Haldir's death. I knew, how badly I wanted Yavanna to be set off, and what I wanted to happen...but the issue was. Whom would set her off to that major level? And the only answer I came up with, was Malifecent.
So from that point of view-aswell as few others. It makes sense :) I think also at that point. Yavanna and everyone else might realize, what Yavanna's purpose is...and why her powers had affected Saeril down in the mines-so truth time, that Yavanna's destiny is to defeat Maleficent. Yeah, no one will be happy that ultimately Yavanna, Saeril will have to battle it out. But shit happens hey?
Yeah, once Yavanna would get over Haldir's death...and Malifecent's doing...she'll need some help on knowing how to destroy that darkness. She'll probably realize why her powers affect Saeril, not long after the battle...realizing her destiny first. Maybe others might have their worries/suspicions, but wouldn't voice them out loud, until Yavanna says something.
I am thinking in Lothlorien, Lady Galadriel will be the only one to know what Yavanna's destiny is-but neither Saeril or Yavanna can read her mind, as she keeps those thoughts blocked from both of them for the time being. But maybe she has a few words with Legolas, to give him a bit of a warning and try to help somehow ensure that Yavanna succeeds.