"What I'm asking is: what is the point of trying to argue that certain kinds of coerced sex are not rape?"
The point is that if start using a definition outside the standard definition, you tend to lose the trust of an outside observer.
Suppose someone wants to determine what happened in a given set of circumstances. If a person says, "He did X to me under Y conditions" and the observer comes back with "but Y conditions would seem to make X rather difficult" and the original person says "well, I have different definition of X", the observer instantly feels like the credibility of the person has decreased.
If a person gives a pretty unambiguous description of events, their credibility tends to be high. If a person's story is going to be widely believe, believed in a court of law and so-forth, we want their credibility to be high.
I think I can understand emotional appeal of the argument that the victim shouldn't be under scrutiny and shouldn't have to prove her case. But, I'm sorry, reality can't work that way - any system that discards investigation into truth will instead wind up with the truth suiting those having the most power and that only guarantees more victimization on one level or another.