making the point that NTR is a general term that covers Netorare, Netori, and Netorase
To my understanding,
NTR = Netorare (i.e., in other words, cuckoldry). NTR is not an all-encompassing acronym that is used to classify the other forms (i.e., those being netori/netorase). So, technically, the OP is not incorrect, as NTR (i.e., Netorare) hasn't occured just yet (and probably never will be implemented).
So in this case, if Omar were to steal away Zahra and fuck with her behind the MC's back that would be considered NTR (i.e., Netorare).
Netori already took place, because MC already cucked Omar
This is correct. Netori has taken place....MC and Omar's wife.
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NTR is a general term that covers Netorare, Netori, and Netorase
^ But this to my understanding is not true, as NTR is not meant to be construed in a general/overarching way by interlinking Netori/Netorase themes as well. NTR is simply limited as an acronym to Netorare which is distinct from the latter two aforementioned themes.
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This was confusing to me at first when I read into the differentiating terminologies a few years prior, but I believe that this is how the meaning is meant to be conveyed.