I don't yet buy the theory, but it's got some interesting support. I don't see it as repetitive, however, because if she is connected to Andruil, it's as a rather unwilling and unhealthy host, which brings up the biggest objection I have to the theory- Flemeth's statement that the unwilling can't be possessed. However we do know that people can become possessed without realizing that that's what they're doing. So conceivably Sera could have heard "Andraste" talking and accepted a bargain of sorts and ended up with an elven god's wisp essence in her who's now driving her mad. This would make her, not a repetition of Solas' story, but a contrast. Solas even gives support to it when he says "she is apart from herself." She's not integrated as she should be.
Again, I'm not saying I buy it, but something is going on with her.
Whatever is going on with Sera is a chance for some exciting storytelling
A hidden mage would be neat, someone linked to a god like Morrigan/Inky after the well would also be a nice fit, or a child suffering from magic-related childhood trauma... all of those are interesting possibilities.
That idea of Sera mistaking whoever approached her for Andraste is awesome, I hadn't thought of that! Sera could be so desperate to prove or find out that the Maker and Andraste are real because she wants to convince herself that what she accepted isn't something more sinister. Also, we've already seen god-like elves who accept and integrate the spirit and power so it'd be neat to see that story told in a different way, with a host that actively resists the spirit. Some may think Sera is irritating because she's an outspoken and grating character, but she's got a lot of strength. If anyone could stick it to a 'god,' she could
Good storytelling, imo 
But really, I didn't intend to stir up dissent or bring the Sera discussion back, all I want is to have a thread to come back to that feels like home and doesn't pick on people who have unusual ideas. We're Solas fans... we have enough on our plate to think about without worrying about how silly a theory about another character sounds 