RJ made very clear that Fain could not corrupt anyone with the Taint of SL (th way he did Toram Riatin, for instance) before he got the dagger. It's not him who spread the taint, it's the dagger itself.

His power to corrupt minds before then was strictly with his tongue, Mordeth's tongue. Niall, Elaida.. he comforted them in their own beliefs about Rand and furthered their mistrust. That is of Mordeth and Aridhol, but that is not the taint of Shadar Logoth.

Elaida would be long dead if she had been "tainted". Like Riatin etc., like it nearly happened to Mat.