Hi,

""""""""""""So I think I understand that left/right will rotate the object in 90 degree steps using a revolve axis that is vertical relative to the current view, and up/down will rotate the object in 90 degree steps using a revolve axis horizontal in the current view.w""""""""""""""

Yes, the axis being centered in the bounding box of the object or selection.

""""""""""""In the 2D Top/Front/Right views the revolve axis should be in a world x/y/z axis direction, right?""""""""""

I'm not 100% certain I understand what the difference would be between, for example, local front xyz and world xyz.

Concerning the 3D viewport it could be the same as the top view, but your idea about the closest to an orthographic view direction is quite good

Marc