Thank you! I checked (more detail below) and the prank did come before the "worst memory" scene, though we don't know for sure that it was during the same year. (So I guess we also don't know for sure whether James was an Animagus yet at the time.)

You're completely right that Snape already knows for sure about Remus (having glimpsed him, transformed, down the end of the tunnel during the "prank") at the point that he's heavily dropping hints to Lily. Funny, I'd vaguely assumed, just based on my memory of that dropping-hints scene, that Snape didn't yet know for sure at that point – he was only guessing based on what he'd been able to figure out himself. (He's "ill"? Every full moon?) But that conversation states quite specifically that the prank had already happened before it. I guess what's going on is that Snape does know, but he's trying to present it to Lily *as if* it's just a theory of his, backed up by quite a bit of circumstantial proof. ("There's something weird about that Lupin. Where does he keep going?") Quite clever of him, actually, given that he's forbidden to tell what he knows!