Neither makes sense/

This is a very interesting question. It is definitely less common than saying "I'm Aaron", but it still makes sense when you take in the context. Here he is assisting customers in the store and in some stores the cashier will ask you "have you been helped by anyone?". In these stores, the employee gets commission (a percentage of the sale) for every sale they help generate. The phrase "I have been Aaron" could be in reference to this. Cashier: "Have you been helped by anyone?" Captain America: "Yes, I have been helped by Aaron." This still holds for other situations where there is no commission. Maybe they want you to give positive feedback on their service, maybe they want you to remember them if you need help, or maybe they just like to say it that way.

The employee said 'I've been Aaron' rather than 'I'm Aaron' because he was using the present perfect tense to emphasize his role or identity as an employee who has been helping Steve and Natasha. He was implying that he was not just stating his name, but also his function or service. For example: I've been your teacher for three years. (I'm not just telling you my name, but also my role or relationship with you.) I've been your friend since we were kids. (I'm not just telling you my name, but also my loyalty or bond with you.) In the context of your dialogue, using the present perfect tense suggests that the employee was trying to be friendly or humorous by highlighting his assistance or availability to Steve and Natasha. Using the present tense would simply indicate his name without any additional meaning.

Are you going through all the Marvel Movies before you see Infinity Wars?

This one is really hard to explain. I'm not sure I can. 1. At some restaurants or stores where you get personal help in the US, the person helping you will introduce themselves by saying "I'm Jane and I will be ________." (with the blank explaining what they are going to do) Toward the end, they might say something like "If you need anything, I'm Jane and I've been __________." (again, explaining what they did). This isn't the type of thing that you would say anywhere else, but it happens in stores and restaurants. 2. The guy at the Apple store is kind of weird. The way he looks and speaks are a little bit strange. Right at that moment, he's taking the store phrase and changing it just a little. So, you have a strange guy saying a strange thing in a strange way. It's a little bit odd. Native English speakers also thought it was a little bit weird (but it fits in with the character). Why would he say it like that? Because Aaron wants to be a little bit different. He knows it's weird sounding. He likes it.