You think she may have been trying to warn them about Solas? As of the epilogue to DA:I, she does know that Solas is awake and plotting something with the orb. She's a smart cookie; she probably worked out his plan before he killed her.
Personally, I don't think Mythal is particularly benevolent; she's clearly got her own agenda, and the way she talks about her betrayal during the dialogue in the Fade makes me very nervous. But she does see the modern elves as "the People," so maybe the dreams were her way of tipping them off. A gesture that requires little effort on her part, but helps nudge them in the right direction.
I'm not sure Flemeth was trying to warn the modern elves about Solas, but I do think it's odd that she seems interested in them when Solas is not. Conversely, she apparently didn't care about the ancient elves enough to let Abelas know she was still alive for all those years, but I felt like Solas was trying to recruit them.
I agree with you that Mythal doesn't seem benevolent which worries me too. In fact I'm just going to say this next crazy thing so that it doesn't come true. Please, please let them not be luring the modern elves to some giant blood sacrifice ritual. Solas made noises about not wanting people to suffer before the end, but it's not like guilt has ever stopped him before right?
Which would give the Evanuris more reason to murder her when they find out she's been aiding the slave rebellion, that's also probably been disrupting their war against the Forgotten Ones.
If the Evanuris knew that Solas and Mythal were working together, wouldn't they make sure Solas was otherwise occupied at the time of Mythal's murder?
Do you guys remember the dialog with Solas about the Well of Sorrows where you can tell him you trust your friends, and he replies 'I know that mistake well enough to carve the angles of her face from memory'? I know some folks here take that as a figure of speech, but I've always thought it was a literal person. Maybe one of the Evanuris pretended to be on their side and then made certain Solas couldn't come to Mythal's defense?