I had trouble entering this fantasy because I can't really picture the gov'ts of the world allowing the Prawn to live in such squalor conditions.
I know, after the movie I had to go and look up international responses to South Africa during apartheid, among other things. Turns out that there was the normal international response: condemnation and trade sanctions. There was no other action.
Would the stakes be raised if they were aliens? Possibly. But you'd be surprised how easy it is for a government to term the expense and difficulties of dealing with aliens and refugees Someone Else's Problem.
Why didn't the Prawn just individually decide to use their weapons to get what they wanted, for example shooting the Nigerians to get Cat food.
I'll have to watch it again. There wasn't much about the pre-slum history of the aliens. Perhaps some of them did. I also have a suspicion that, despite their advanced weaponry, a lot of them were intentionally non-violent towards humans. Perhaps the ones living there after even a few years wouldn't want to risk a large-scale government response by violent action, and possible recrimination on all of the aliens. You'll notice the armed guards near the beginning of the film, when they go to serve eviction notices. Even with the confiscated weapons, they're still suspected.
Also what were Nigerians doing in South Africa?
Black market crime lord. If they could get the weaponry to work for them, imagine what they could do in Nigeria.
If Christoper was a leader Prawn why didn't he rally the people?
Again, something I'd have to watch the movie again to give a better reason for. We don't see the early history of District 9, so there may have been a previous leader figure.
But imagine, if you will, what would happen if a powerful leader emerged among the aliens. Do you think that would be remotely in the interests of some of the other aliens, let alone MNU or the black market? MNU doesn't want to deal with an organized populace. Nor would the black market Nigerians. They'd probably soon assassinate him. He's one of the few (if not the only) who can figure out how to get the fuel. He can't jeopardize himself by being a leader.
Why does alien fuel turn people into aliens?! It makes so sense.
If it's based on biological agents, then it might also include retroviruses which would ensure that the DNA (or its equivalent) does not degrade, rendering the fuel useless and inert. Thus Wikus just got sprayed with the retrovirus which is modifying his DNA, first replacing damaged tissue and gradually his healthy tissue.
Also if their weapons are powered by some sort of biological agent/DNA then why couldn't they just use lower lvl Prawns as fuel?
I assume you mean MNU. In answer, they probably already tried to get the weapons to work that way, but it wouldn't work with a human at the trigger.
Or you could mean Christopher. In which case... how monstrous do you think they are, killing their own to generate fuel? Not to mention that it probably wouldn't work, since there must be some refinement process that Christopher wouldn't be able to do with human technology. He could only work with the alien technology's already-refined liquid product.
Why wasn't the scene from the trailer in the movie?
I thought the scene provided good backstory. It was probably made before the final cut. And perhaps they didn't mean to ever include it, since it'd be tough to include an initial interrogation scene which makes the aliens look sympathetic in a documentary that would have no access to such footage.
Why did the movie end halfway through the story?
It did? As I recall, the two aliens got away safely and MNU was exposed, thus completing that part of the story. There is more to say, but it was a good place to end it. See next question.
Where is the sequel?
Blomkamp is very interested in a sequel, after he does another project or two.
A Multinational corporation would never be delegated to handle a brand new intelligent species.
What government wants the burden and responsibility? What NGO has the resources? Somebody else's problem.
Why was their inter racial prostitution if all the Prawn were A-Sexual?
There are many ways to have sex. And the one being paid doesn't have to enjoy it. (Conversely, it doesn't have to be reproductive for someone to enjoy it.)
Where were the female Prawn?
If the aliens are asexual, why would there be females/males? We are the ones that gender them.
If a mother ship gets lost in space/stranded, no body would come looking for nearly 3 million people?! Why didn't the home planet send a search and rescue?
Any number of reasons. Maybe they didn't have the energy to send a distress signal. Maybe it was too far away for light-speed communcations to reach them. (If it'll take three years to get home, the ship must be traveling at FTL speeds.) Maybe their communication devices weren't strong enough. Maybe the home planet didn't know where they were lost?