Why wouldn't Mythal help out the People?
Well, if she was in wisp form most of the time until Flemeth, Flemeth was born at the beginning of the Age after the fall of the Dales. And the Andraste theory would have it so that she tried to help them gain a new home and end their slavery by teaming up with Shartan.
I get the impression that Mythal can't act too openly for whatever reasons. She gets heroes in debt to her and uses that to manipulate history. It looks like she's gathering power. Hell, from the amount of blue light in the epilogue, I'll bet that Urthemiel isn't the first powerful soul she's snagged. (Maybe it's the other daughters? Other Old Gods? *shrug*)
I was...vexed that Mythal didn't explain things to a Lavellan any better. I appreciated Abelas's bluntness in, "Shadows wearing vallaslin. You are not my People." He's lived for thousands of years living the actual meaning of the vallaslin, I can't blame him for being sore.
Mythal was the mother guardian goddess, everything went to hell when she was murdered. She seems to hold some fondness for the Dalish, though. When she's resurrected in 2, I originally thought her comment to Merrill:
"Do you know who I am, beyond that title?"
"No."
"Then stand. The People kneel too quickly."
Was an epic burn against the Dalish that actually shocked me with how nasty it is. Now it comes across as a mother admonishing her children for being too easily impressed.
Solas is excusable that he doesn't tell, for all the reasons other people have mentioned. What isn't excusable is holding the Dalish in contempt like he does, when his own actions had a role to play in the fall of Arlathan, the enslavement, and the fall of the Dales. Fine, some Dalish got snippy with him when he tried to reveal the truth to them. Lavellan is not them. I do appreciate that he tends to get flustered and apologized whenever you point out, "We might get it wrong, but we're trying, and we can't walk the Fade for answers like you do. You're looking down on the People for something that only you can do."
SOLAS: Right, right, Dreamers aren't a dime a dozen. *sighs*