-Why in the north of the world (For example nordkapp) it's always night at a time of year? I have seen that with my own eyes. This zone is in the "center" of a flat earth. 
As I understand it, the sun circles above the earth but as the seasons change the circle expands or contracts. During winter in the northern hemisphere, the sun circles in a circle wider than the equator, so it is far from the north pole. The light just doesn't get there.
-What are northern lights and why occurs only in the center of the flat earth and in his edge?
I don't know what the auroras are exactly, but it is interesting that they only happen close to the center and at the edges of the earth. I suspect something to do with magnetism but I am not an expert here at all.
-Why compasses point magnetic north?? And why magnetic north changes?
-Why a flight duration between London and Toronto is the same (8 h) than between Luanda and Sao Paulo? The distance of the second flight should be biggest because this cities are more remote of the center of the circle.
There are several explanations as to distances in the southern hemisphere, but for one thing, there isn't an accurate "map" of a flat earth without distortion. until we have one, there are bound to be discrepancies like this.
-Why I cant see the edge of the world?
Are you in antarctica?

It's pretty far away. I don't think the edge is where most people think it is (I suspect there may be something "past" antarctica), but that's just a hunch on my part.
-Why does the water fall spinning in the other direction in the southern hemisphere?
It doesn't, this is a myth.
-Why the earth is flat but the other planets are spheres?
Probably the same reason that the earth has human life and the other planets don't. The earth is clearly different.
-How can the satellites orbit arround the earth? How works GPS?
I think answers here will vary. i think if the sun and moon can circle above earth then satellites probably can too, but I don't know what holds them there.
Sorry if I was vague or uncertain in my answers, I'm new and questioning a lot of things myself.