I'm not sure about all that, but okay. It mostly sounds like it would be prudent to just kinda pick a definition like "The POV character experiences humiliation as their significant other is claimed sexually by another", or whatever, and if something strays too far from that then it doesn't count. If a 10 people have 12 different definitions for a term, it's essentially useless as a word. It doesn't sound like there's infinite nuance, just a situation where people haven't figured out new terms and tons of people are using the same word for very different things that just so happen to have a little overlap.
It's accepted that the one being stolen in NTR is the female, whether she is the MC or not. Reverse NTR is when the male is stolen. Also, she doesn't have to be in a romantic relationship. They just have to share a deep bond, whether it be unrealized love, platonic love, familial bonds, etc. That bond is broken by the end of the corruption arc.
It can get murky when you get into the cheating vs NTR with a female MC. We can say that NTR deals with more of the psychological, the corruption process, and the fact that the female is eventually
stolen. There are always outliers, whether it be because the tag was used loosely/incorrectly or some other reason, but the general vibe of Japanese NTR stories almost always revolves around the concept of the female being corrupted into cum dump for the baddy.
It does start to lose meaning these days when pretty much everything involving a non-MC male is considered NTR.
The whole jealousy and humiliation aspect that people swear by is more of a western construct stemming from the cuckold fetish. I mean, Japanese NTR can also have those themes, but there are so many NTR stories where the male protagonist doesn't even know about it until the very end (and sometimes never finds out). It's the journey of the female character that is the more important part, regardless of the POV.