Iona has a daughter, her husband died of disease, she dies because of Howe's attack. So Sera would be still be an orphan. Lady Emmald could have been a friend of the Couslands took her in, when Sera found she was a ******, Sera ran away found her way into the Red Jennies. It's IMHO more probable than her being like Solas or Flemeth.
I never said that I bought the Andruil theory. It's just in-the-air speculation that's based on inference and "hints" drawn from various places. It might be true but in my opinion there's no compelling reason to believe it.
Your theory, however, flat out contradicts the facts as we know them. Sera was an orphan before she was taken in by Emmald which is years before the blight. She doesn't remember her parents. Sera was actually left Emmald's estate in her will when she died (also before the blight) but turned it down and ran away (this is explained in World of Thedas vol 2). She makes it fairly clear that she lived on the streets after she left Emmald but not the alienage. At some point she joined up with the Jennies.
Iona's daughter, who is called Amethyne, knows who her mother is and lives in the Alienage. She is seen in Denerim in Origins wondering when her mother will return. Clearly not the same person, sorry.