Why did the aliens not retaliate against the oppression from mankind?
Although many aliens participated in riots and general upheaval, it is assumed from the documentary portion of the film that there was no actual alien-led attempt at fighting back and gaining freedom from the humans until Christopher Johnson and Wikus Van De Merwe (Sharlto Copley) teamed up. There are several possible reasons:
1) At the beginning of the film, it is mentioned that the aliens aboard the mothership may have been mostly "worker" aliens, who by nature or design lacked the qualities required to rebel. Without a leader, they didn't know what to do and were basically brutish.
2) They were found malnourished and very sick, and so they were in a vulnerable situation and very easy to subdue. The few that had weapons had to be very secretive about them and were possibly waiting for a better moment to strike.
3) Many were essentially crippled by an addiction to cat food, which may have affected them in the same way as crack cocaine does humans (a reporter draws a comparison to cats and catnip).
How did the "prawns" come to possess their highly-advanced weaponry?
It's made clear in the movie that the aliens have no way back up to the ship, and they only came down with the assistance of the humans. Therefore there's the obvious question of why the aliens have so many weapons in the slums. The movie does not address this. Some options:
1) The weapons are taken from the ship by aliens when evacuated, and are not confiscated. Plausiblity depends on who is responsible for the evacuation, how organised the evacuation is, and how obvious it is that the items are weapons (e.g. stowed/unstowed).
2) The weapons are taken from the ship by humans at any time, or are confiscated from the aliens by humans during the evacuation, but wind up back with the aliens somehow. Plausibility depends on who holds the weapons, when the weapons are stolen, who steals the weapons and why, and how the weapons are passed back to the aliens.
3) The weapons arrived on the dropship. Plausibility depends on the number and type of weapons. Certainly there was room for no more than a few of the Mech suits in the dropship.
4) The weapons literally fell from the sky off the mothership, like the other garbage items which Christopher scavenges, and were repaired. Plausiblity depends on where the weapons fall from and where they land.
5) The weapons were discarded by the humans on the assumption that they do not work. They may not have been aware that the weapons work when an alien holds them. Plausibility depends on where the humans discarded the weapons.
6) The weapons were brought in pieces, unrecognizable to humans at the time, and reassembled later, much like Christopher taking a few pieces of what looks like scrap metal from MNU and snapping together a bomb in a couple seconds.
7) They recreated the weapons from memory or instinct using scraps from the ship and available human technology.
Why are space aliens hosted in an ordinary slum instead of quarantined and investigated?
In any other science-fiction movie, when alien life or artifacts are encountered, the area is quickly quarantined, and military and scientific personnel in hazmat/biodanger attire proceed to investigate, take samples, and so forth. It is highly unusual for a large number of aliens, who were also sick and therefore an increased biological risk, to be left wandering in a slum in the outskirts of Johannesburg. It's odd that no other nation seemed interested in researching mankind's first encounter of non-human intelligent life and their technology.
Explanations that viewers have offered include:
(1) Due to the Cold War (the aliens "landed" in 1982), neither Russia nor the US dared to make any overt attempts at securing alien technology,
(2) The 28 years between their arrival and the present day of the film is described only briefly in the documentary footage with little to suggest how they were transported to Earth, whether they were initially quarantined and examined, and when it was determined that they posed no real biological risk,
(3) The Cold War and the extent to which South Africa was sanctioned and ostracized by the international community over its apartheid policies might have influenced the lack of international intervention, and
(4) It is quickly disclosed in the movie that the aliens are sick, malnourished and relatively undeveloped, making them more of a nuisance and liability than an asset. This may have reduced scientific interest from other countries; because of our 'traditional' view of an alien civilization, humans try to instinctively avoid much contact with what could be perceived as 'hideous' or 'inferior' beings.
(5) South Africa is not as developed as many other western countries. This combined with the fact that humanity was unprepared for an unexpected encounter with an alien civilization could mean South Africa was simply not equipped to handle the crisis and hence dealt with the situation poorly. Other nations did not get involved due to legal and bureaucratic reasons.
