With the magic level in decline, a Domain could be something that's considered a Mana Ebb, or Void in SR. During the downcycle the Mana level was so low, that it didn't allow any magic to those who didn't know it, leaving only legends from a previous age. And as for Avalon, i bet that's a leftover from the Age of Legends that the IE's brought with them. As most legends probably are in the Sixth World.
As for Harlequin, i'd stay with a Magic score of 25 and Initiate grade of 20. It's high above anything the players can get, enough to take a Great Dragon head-on. Game balance wise it's fair, looking at the GD's statistics in SR4A*. Though still Dragons aren't bogged down by the game mechanics - they wish spells into existence in a dynamic way*. And i'd add at least a few Initiate powers to any dragon, Great or otherwise, as they had to learn a few things eventually.
* Though i think some dragons, like Big D, or Ghostwalker, who were renowned for their magical skills in the AGe of Legends, have their magic scores way above 12. Probably around 20-25.
** Just like in Mage: The Awakening, only they mostly improvise. Being a creature of magic pays off...