I don't know--Snape himself draws attention to Harry if there's a possibility of embarrassing him. As with the Skeeter article he reads aloud in class, I bet he was delighted by it.

I think you're right on about the Snape and Draco interaction, though. I don't think Snape would've overtly helped Draco create the valentine, but I think he would subtly encourage Draco's plans. Plausible deniability, you know.