I would think both 'we leave now.' and 'we are leaving now' is tranlated with 'nosotros salimos ahora' , thus, my answer 'we leave now' should be correct?
I think your right with your first point but "we leave now" isnt propper english.
why isn't 'we leave now' proper English? if someone asks 'when are we leaving?' a person can say 'we leave now' i leave, you leave, he leaves...etc