I don't think the conclave is supposed to be a secret (even the carta hear about it), and there are a number of ways Clan Lavellan could have heard about the meeting, from trading with humans in the Free Marches who heard about it to elven mages from the Circles seeking sanctuary with the clan. Sending a single, trusted Dalish probably helped the First or hunter reach the conclave in time.
See, I was going by this quote from the website: "What happened there, she said, would impact not only the Dalish but indeed all elves. She could not have known how right she was." Combined with what was being discussed in the old threads when this info was first released, I assumed that the Keeper actually did have some kind of mystical insight and that the impact on elves had to do with magic... like she had some idea of what would happen with the Fade tear in advance.
At this point though, I think it's probably more likely that the Keeper was concerned with the more mundane aspects of the Mage-Templar Conflict... that's certainly the simplest explanation... (edit: and of course, the Keeper wouldn't need to be in contact with spirits to know about any of that...)