Yeah he failed at watching over her as we know he let her be burned and the Warden can destroy her ashes so he if he is watching over her he has been doing a pretty terrible job at it.
Watching over is different from actually doing anything.
Whatever the case is, Andraste's Ashes had unusual healing powers, no matter how Lyrium infused they might have been. Lyrium doesn't have healing powers on its own.
All of the manifestations in the Temple were extremely unusual, if nothing else. Even in a Lyrium rich environment you wouldn't normally expect things like that.
I'm not saying that the Maker definitely exists, and certainly not exactly as the Chantry imagines him, but I'm willing to believe that Andraste had a way of getting Lyrium and magic to respond and do things that most Mages couldn't. This ability may have caused the Lyrium around her final resting place to develop and grow in an unusual manner.
Andraste had unusual knowledge of historical events. Knowledge which turned out to be more or less accurate. She supposedly had miracles backing her up, and against a magocracy like Tevinter she must have had some good advantage working in her favor. If she could communicate with Lyrium in some way through her singing...
Well. That would be just the sort of advantage she could use, and it could be just the sort of power that could influence the Lyrium around her remains. Whatever the case, the Lyrium in that mountain was not acting how Lyrium normally acts.
This will probably be a very important plot point, because not every Fade explosion will result in a mysterious sole survivor who met a magic green woman in the Fade and got a magic hand.
As for the Elder One himself, he's already defined himself by one trait. His age. There's also a subtle difference between being Old and Elder. Old Gods are Old Gods because they were worshiped in old times, by the old magisters of Tevinter.
Elder can mean much the same thing, or it could mean that the described entity is older or even derives authority from their greater age.
The feeling I personally get from the title "The Elder One" is not of something that is simply just very old. To me it feels like a something that is perhaps older than the rest of its surviving kind. If he's a Demon, he's probably the oldest Demon there is or a contender for that title. He's a being that may have seen the Golden City whilst it was still Golden, and knew a time before Thedas was separated from the veil.
This being having an army of demons at its command is... Something. Aren't Demons usually territorial?
Now, in interviews its been stated that the Elder One is so arrogant because he feels he's the only one who really knows what's going on, like he has a higher understanding of events than mortals or even Spirits do. So I imagine that's probably related to his age, but there's probably more to it. He's probably aware of events that relate to Andrastian beliefs, but even that wouldn't necessarily explain his attitude in its entirety.
However, even the idea that there was a time before the Veil, when Thedas and the Fade were one is pretty common knowledge, even if some people act surprised by it for some reason. So. What does the Elder One think he knows that he's so damned smug about?
I like to think the Elder One changes with the Inquisitor's class so it is a fair fight. Mage death match. Rogue Death Match and Warrior Death Match.
As I said, you're obviously the Doppelganger and the Elder One will be possessing the original you.
It may be a joke, but that doesn't mean it can't be true.