Shishi: A Tallis may only be put on top of a Sefer Torah in a case where it’s a Kavod for the Torah (meaning it would be a Bizayon to be left sitting uncovered).
Interestingly enough, your comment made me realize that public Taleisim in Shul probably also have a Din of Tashmishei Kedushah because they are occasionally used to cover the Sefer Torah.