And it's the ashes that get me. Sure, a Joan of Arc figure could rise up under the banner of a god that isn't there and then get canonized/deified herself, but for those ashes to have actual healing powers (not to mention all the other stuff that went down in the temple during DA:O) without some kind of deity at work is quite a stretch.
We know that Faith Spirits are Hax as far as Spirits go. Beings from a dream realm fueled by belief.
Spirits can either take the form of people from their observations and from what they learn from their memories, or an intense enough death can lead the dead person's identity to become imprinted on Spirit whose nature resonates the most with the deceased at the moment of their death.
At it's most basic Blood Magic can refer to either creating a sympathetic bond between the blood and it's owner(stuff like tracking, or voodooing them) or sacrificing life-force to power magic which weakens the Veil.
Andraste as she burned, basically got Sacrificed and that combined with the belief in her Divinity no doubt served to heal Havard.
The Location of the Temple, built upon a Temple to Mythal, the Gauntlet closely resembling the sort of puzzles and challenges you face in the other Temple of Mythal.
The amount of Disciples of Andraste that Sacrificed themselves to pyres in the Sacred Ashes Temple, I mean, literally lit themselves on fire as a show of devotion to Andraste.
High concentrations of Lyrium.
As far as I'm concerned that's the first time I've read of putting an Epic Level Enchantment on an object, by accidentally using Blood Magic and Consorting with Demons.
The Lyrium and Blood Magic provide fuel for the Enchantment, Belief and Faith Spirits Direction, the Temple already has kick-ass defenses and who knows what else, the Ashes' one of healing power becomes a permanent enchantment, it could even be that the Urn was enchanted and would have always been full.
The Sacred Ashes are nothing more than Demons, Lyrium, and Blood Magic.