Solas want open fade he actually tells you in game. He is fascinated about fade he want's to restore what's lost
"Imagine if spirits were not a rarity but a part of our natural world like... a fast-flowing river. Yes, it can drown careless children, but it can also carry a merchant's goods or grind a miller's flour. That is what the world could be if the Veil were not present. For better or worse."
So basically Solas want same think as Cory remove veil. Except Solas is good, he don't want become god and people die.
He gave Cory orb to open it because he can't. Solas fought explosion would kill Cory, and it did, but one small problem Cory is immortal. So he joined Inquisition to fix his mess.
"No real god need prove himself. Anyone who tries is mad or lying."
1. Flemythal send something through eluvian, maybe part of her or her soul to Morrigan.
2. Most likely Solas took raw power from Mythal what's left of her. Maybe she gave it freely or Solas steal it we don't know much speculations not much facts.
