Accepted Jan 19. 'Come back soon' is a ready made sentence which conveys the sense of the Spanish. I am not sure 'return soon' does. It could mean go away.
Usually mister is paired with a surname in English. But not always. For example, in these song lyrics, "I come from down in the valley where, mister, when you're young" (The River, Bruce Springsteen) and "Listen, mister, can't you see" (The Letter, The Box Tops). However, in these instances the usage is very informal if not slangy. For this exercise, I don't think mister strikes the right tone. I suppose Sir is best.
It's the other way around. The verb is volver, so vuelve is the informal imperative, and vuelva is the formal.
quickly = rápido (it expresses how he should return)
soon = pronto (it expresses when he should return)
True, but my "Larousse Dicionario de Sinóminos Antónimos" does list "rápido" as the first synonym for pronto.