Buz you and the others are arguing past each other.
What we have is a measure made that is supposed to indicate overall happiness of a population. The results of this indicate that Nigerians are, by this definition, HAPPY.
This is in spite of all that you have said. All you have said may well be true. But all we have to measure some sort of average happiness is this survey.
There is no good being incredulous. I find it surprising too. The only thing that makes sense is to more closely examine the nature of the measurements made and the methodology. Since we don't have access to that I think we don't have anything to discuss.
The other approach might be to find other surveys of "happiness" that have been made. What is it that correlates with happiness. We all suspect that material wealth is not well correlated with happiness. Is living in a peaceful country? I imagine it is, as you do. However, maybe it doesn't correlate well. If that were true in other surveys then the results of this survey aren't so surprising.
We might also, as you seem to suggest, examine who the people were who made this survey, do we think they have a specific axe to grind? Why do we? (other than simple paranoia) Motive? What good does it do to give results like this? How are they being used? What other countries are given? What do we think of the over all results.
I have certainly read a bunch of anecdotal stuff were individuals marvel at how happy impoverished, downtrodden people are. This, of course, proves nothing, but indicates that it is not impossible to be happy under conditions you and I might find appalling.
Without all this I don't think there is much use of further discussion on either side.
All we have is that someone told us a study says Nigerians are very happy. We have no idea about the reliability of that. However, we do feel that it is not impossible even if we individual are very surprised.