Great OP! really useful information and theories!
However im pretty sure it was Andruil who made the first Halla?, not Ghilan'nain. Andruil saved her when she was lost by turning her into a "deer" which she then named Halla, helping her find her way back home.
We have two conflicting versions of Ghilan'nain becoming a god.
The one your referring to from the dalish:
They say Ghilan'nain was one of the People, in the days before Arlathan, and the chosen of Andruil the Huntress. She was very beautiful—with hair of snowy white—and as graceful as a gazelle. She kept always to Andruil's Ways, and Andruil favored her above all others.
One day, while hunting in the forest, Ghilan'nain came across a hunter she did not know. At his feet lay a hawk, shot through the heart by an arrow. Ghilan'nain was filled with rage, for the hawk—along with the hare—is an animal much beloved of Andruil. Ghilan'nain demanded that the hunter make an offering to Andruil, in exchange for taking the life of one of her creatures. The hunter refused, and Ghilan'nain called upon the goddess to curse him, so that he could never again hunt and kill a living creature.
Ghilan'nain's curse took hold, and the hunter found that he was unable to hunt. His prey would dart out of sight and his arrows would fly astray. His friends and family began to mock him for his impotence, for what use is a hunter who cannot hunt? Ashamed, the hunter swore he would find Ghilan'nain and repay her for what she had done to him.
He found Ghilan'nain while she was out on a hunt with her sisters, and lured her away from them with lies and false words. He told Ghilan'nain that he had learned his lesson and begged her to come with him, so she could teach him to make a proper offering to Andruil. Moved by his plea, Ghilan'nain followed the hunter, and when they were away from all of her sisters, the hunter turned on Ghilan'nain. He blinded her first, and then bound her as one would bind a kill fresh from the hunt. But because he was cursed, the hunter could not kill her. Instead he left her for dead in the forest.
And Ghilan'nain prayed to the gods for help. She prayed to Elgar'nan for vengeance, to Mother Mythal to protect her, but above all she prayed to Andruil. Andruil sent her hares to Ghilan'nain and they chewed through the ropes that bound her, but Ghilan'nain was still wounded and blind, and could not find her way home. So Andruil turned her into a beautiful white deer—the first halla. And Ghilan'nain found her way back to her sisters, and led them to the hunter, who was brought to justice.
And since that day, the halla have guided the People, and have never led us astray, for they listen to the voice of Ghilan'nain.
--From "The Tale of Ghilan'nain," as told by Gisharel, keeper of the Ralaferin clan of the Dalish elves.
And the one from the temple of mythal which tells a story of a poweful being who created many creatures (including the halla) and monsters who were so fierce that she was offered a position amoungst the Evanuris, "godhood", as incentive to destroy them.
Ghilan'nain kept herself apart from the People. She used her power to create animals none had ever seen. The skies teemed with her monsters, the land with her beasts. Andruil hunted them all, and after a year of killing, approached Ghilan'nain with an offer: the gods would share their power with Ghilan'nain, but only if she destroyed her creations, for they were too untamed to remain among the People. Ghilan'nain agreed and asked for three days to undo what she had made.
On the first day she struck down the monsters of the air, except those she presented to Andruil as a gift.
On the second day she drowned the giants of the sea, except those in deep waters, for they were too well-wrought, and Pride stopped her hand.
On the third day she killed the beasts of the land, except the halla, whose grace she loved above all else.
This is how Ghilan'nain was made youngest of the gods.
—Story of the elven god Ghilan'nain, author unknown
I think that an original mural from the time when the Evanuris reigned is probably closer to the truth than the pieced together Dalish story. Though still probably not 100% accurate since it is, after all, god propaganda.
The fact that Andruil, presumably with the agreement of rest of them, would offer something as vaunted as Evanuris membership at all speaks to how powerful Ghilan'nain and her creations must have been.