Apartheid South Africa could be considered ideologically extreme, yet was the status quo. The norm is not necessarily the centre, and it is not necessarily benign.
That is a very good point. And that does occur more often than we would like to admit. In which case the ideology that's being labeled "extreme" is being labeled by another set of extremists that have nevertheless gained control of society, and the labeled extremists may in fact be the 'norm' reasserting itself.
Extremist is a relative label though. It is only 'informative and accurate' to those who share aspects of a common ideology.
Yes, well, that would be true of all 'information', though. Information that cannot be translated to a shared experience or understanding of reality doesn't inform us of much.
At what point does someone become a left or right wing extremist? A Muslim extremist? Extreme nationalist? All of these depend on your start point on the spectrum.
At the point where they threaten the status quo. The term "extreme" still means something even when it means something relative to something else. In fact, most of the terms we humans use only mean something relative to something else. Think about it. Relativism is a defining aspect of the human condition. Up (down), here (there), now (then), right (wrong), and so on.
Terms like fanatic and extremist are equally subjective.
No, they are relative, not subjective. There's a difference. In this case what your calling "subjective" is not subject to an individual human perspective, it's actually subject to a "collective" of humans perspective. That's an important difference.