If you can suspend your disbelief to that degree then good for you. Prometheus is certainly a movie that demands that of the viewer to be enjoyed.
Millburn was not being overconfident. He was acting in a way that would be totally retarded for a middle-school student, yet alone a qualified scientist of any level!
The Marines in Aliens were chosen because they're Marines. Their job was to shoot stuff if needed. The whole point was that they thought they were the best of the best, and their overconfidence was a key element of the story. In Prometheus it is implied that the crew are - at the very least - competent. The fact they turn out to be bungling clowns was a result of the poor scrip/direction and not intentional.
In short, the characters in Alien and Aliens never acted out of character. In Prometheus they constantly led to me scratching my head.
this is where my interpritation differs, based on the conversation vickers has with shaw and holloway in her lifepod, we discover there is a difference of opinion on the missions goals between what shaw and holloway want and what weyland wants. Vickers brings up the issue of funding, its an expensive mission, and you have to remember weylands goals and his issue of time, he also has an android onboard who's iq likely dwarfs anyone else's on board, dont forget his arrogance in this area aswell, he openly tells the crew that david is the closest thing to a son he will ever have, but fails to mention that vickers is his daughter, and he relies entirely upon david for anything of importance.
Based on all of this, it would make sense that the scientific part of the crew was brought on board to help keep shaw and holloway happy, would they have agreed to the mission had they known weylands true goals?, so why would he waste lots of time and money on the best scientists when he has no interest in them and doesnt require them for his goals, he just hasnt got the time for a fully fledged science mission. And to convince good scientists to sign up for a multiyear mission without ever telling them what the goals were, he's either got to dangle a big carrot infront of them, or just go for whoever is willing to take the paycheck, fifield backs this up by saying he's just there for the money.
Weyland would need shaw/holloway on board so ultimatly the rest of the science team was just put together so they would go along with it, it becomes pretty clear they arent needed, which they themselves realise hence leaving the first expedition after fifield gets angry. They are supposed to be bungling idiots, fifields fiery and unfreindly nature plays out with him getting angry and leaving the group, and millburns open and freindly nature, displayed by him trying to befreind fifield, the first thing he does is reach out to shake his hand, whilst fifields holding a bowl of soup and looking rather closed off, yet despite fifields refusal he continues to stick with him, this nature is carried over into his attitude towards the snake, where most people would simply walk away and ignore fifield, he continues to hang around with him, he should walk away from the snake aswell, but choses to try and get close to it, its supposed to be in his nature, and he thinks its mesmerized so gets closer, the deleted scene where they find the smaller eel like creature would have helped here because he isnt afraid of picking it up, and does so without problems, when he finds a bigger version he is confident he can handle it. Ultimatly weylands single mindedness in persuit of his goal and arrogance is the undoing of the mission and it plays out in different ways in the movie and this is one of them.
