I might want the default pfp again sometime in the future, so I'm curious, is it possible to set your pfp to the default one without uploading the default pfp?

Unfortunately, I don't think there is. As you said, though, uploading is always an option. The picture is pretty easy to find. There's no real difference.

No, there isn't. The only option is to either keep the pfp so you have the “true” default or just upload the default.