[Spoilers All] Solas x Lavellan Theory

Everyone is saying that Solas only glows in trespasser to a romanced quizzy. Could he have put mythal into her and used her as mythals vessel? (Please don't kill me for my idea.) I know that vessels can only be used if they are willing, but she is at a vanularble stage. And also, everyone says Flemeth threw the old God soul threw the eluvian . But not everyone has a world where morrigan has a son or a son with the old God soul in general. So? Idk..why does he only glow when he sees a romanced quizzy?