I would be far more likely to say "I will be very quick" (or "I'll be very quick" or "I won't be very long") that the other options you give in the context of a hurried dialogue like this. As for the extra question, I would say the present perfect versions are slightly better English but people often use the alternative versions, especially in informal speech.

"I will be very quick" is perfectly correct and natural. You could also say, "I will be very fast." The two proposed alternative sentences you ask about are also fine. "It was THE most beautiful movie I ever saw" versus "It was THE most beautiful movie I have ever seen." If you use the present perfect, it is clear that the movie is the most beautiful one you have seen until the present time. If you use the past simple tense, it is unclear if the movie was the most beautiful movie you have seen until the present time, or was the most beautiful one you had seen up to the time you saw the movie. So, if you mean up to the present time, it is helpful to use the present perfect. If you mean up to the time when the action occurred in the past (that is, the time you saw the movie), I suggest you use the past perfect: "It was the most beautiful movie I HAD ever seen." The same principles apply to the sentences about THE toughest choice.