Considering the guardian was alive for how long... i say lyrium exposure to that extent cause a biological immortality effect
Yeah but Andraste is supposed to be dead , there's no reason her soul would be tied to her ashes.
Chantry lore says she's at the Maker side.
Tevinter magisters says they saw her in the fade a couple of years after she died.
Now the nature of the fade is a bit funny , since people thoughts can make up things in there...
The thing is we've been told one of the theme of Inquisition is faith .
So I think the Inquisitor will be able to look at things from the point of view of faith , yeah it was Andraste , she left the Maker side to give humanity a chance to survive.
Or no faith , it was just a random spirit , or the soul of the Divine , something not divine .
I mean the moment Andraste enters the pictures and says "Hi it's me" , it's divine intervention and there's not a lot to debate.
As far as I know souls don't linger (unless you do your Flemeth , the ghosts we see in game , it seems ,are confused spirits a bit like Cole ).