The deathless mother questions

Why did fringlia and the elf queen instantly buy into what the deathless mother was selling? I feel like they should have known (based off the visions they had-baby dying her losing power) that clearly this trade off was not going to be in their favor?

Wouldn’t fringilia have learned of her at Aretuza? Not sure why yen was the only one who knew she was evil and said no at first