There's a lot of hints he might actually succeed on some level, but for now all he (and we) know, is that it's likely to be another cataclysmic event with a lot of death. But if he achieves nothing more than flip a switch long enough for the Evanuris to escape... This sounds like something Mythal would want. I think she's more interested in revenge on her old colleagues than bringing back the ancient world wholesale. If the Evanuris return without the veil, they would probably be unstoppable. But if the veil remains in place, they won't be quite as powerful, and it might be possible to destroy them. It still sounds like a massively bad idea.
I looove this concept. I know Mythal says "for a reckoning that will shake the very heavens", and thats fine. It might very well shake them, temporarily.
But Mythal does seem firmly bent on vengeance (odd, since Elgar'nan is supposedly the elven patron of Vengeance). That being said, the Evanuris killed her, however it is one kills a god (likely destroying her physical body? Trapping her in the "Earth", away from the Waters of the Fade?).
Solas seems to think the only fitting punishment for Mythal's murder is an eterenity of imprisonment (though he says Torment I doubt they are being really tormented).
Mythal may disagree. Perhaps, to Mythal, the greatest punishment is that they be cast down from being Gods. That they be forced out from the Waters of the Fade, from that ancient, primordial energy. That they be cast down to earth, lose their powers (Due to the Veil still being in place, perhaps, along with some other unknown parts of her plan), and see how far their followers have fallen: And then be revealed as fakes, false gods, and lose everything. Their powers, their status, their followers...
But, at the same time, all the prophecies about the fade spilling out or the sky tearing open and all the magic being loose again are very compelling.
I suspect Mythal is after vengeance, but I wonder what form it will take?
I wish we knew more of the Evanuris.