If the talking head speculations in the film are to be taken at face value, the aliens may be invading in the sequel. That is not a bit interesting.
The caste speculation assumes that alien castes would be real biological entities, and the bulk of the aliens really are stupid and vicious. Castes in human society are merely social constructs and there is no biologically inherent distinction between members of castes. This is actually an excellent example of some of the confusions in District 9. The other notion, that the aliens are (mostly) sick, might be read as a metaphor for racial oppressions damaging racial subcultures. Leaving it open, and leaving a special exception so conspicuously unexplained rather undercuts that. Which is also an example of District 9's confusions.
The weapons are not paperweights if someone figures out how the safeties work. How any government can be expected to assume the Nigerians might not serendipitously discover the answer is an unanswered question. Truthfully, the implication is that the Nigerians are too stupid. I don't think you want to go there. Again, this is an example of some of District 9's confusions.
And there were a hell of a lot of paperweights for such nice people as Christopher Johnson. I always had trouble believing that Christopher would be so shocked that the MNU could be experimenting on the aliens. It occurs to me that maybe he was shocked that they might be able to unlock the technology. His sudden urgency was to forestall human success in gaining superior weaponry!