Is there an official direction for scroll zoom?
If natural scrolling is disabled, scrolling up zooms in in all of those three cases. Scrolling up zooms in the screen even if natural scrolling is enabled though.

Scrolling while holding control could be considered to be a separate gesture where enabling natural scrolling does not invert the scrolling direction. When using a multi-touch surface, I think it's more natural that dragging up increases the zoom level.



It's also possible that applications shouldn't invert the scrolling direction when scrolling is used for zooming, but many applications do. But I didn't find anything about it in the HIG, and I don't know any applications by Apple where scrolling while holding option or command would have a special meaning.