I really liked this post, OA.
The best way I can get my head around an intellectual description of Ni is to see it as choosing a perspective, not because its factually the most correct or evident, because it seems the most consistent with some internally sourced archetypes.
This is so close to being dead-on accurate. It only has one flaw, which you can see if you compare it to Si. Si references internally stored archetypes, too. However, Si doesn't CHOOSE them: they were created by experience.
Similarly, Ni doesn't "choose" the perspective it perceives, as much as it might seem like it to others. Rather, think of both Si and Ni as libraries of experiences. Si stores the concrete, literal versions of those experiences, but doesn't tend to keep functional/purpose/meaning-based attributes of it. Ni stores a more functional/purpose/meaning-based version of experiences, but tends to lose track of the concrete specifics.
Both Si and Ni can quickly pull up their respective libraries, and quickly rifle through the index to pull up the most relevant past experience.
So what Ni does is look at reality, e.g., at a problem, and immediately sees "what kind of problem" it is, pulls out the Ni-experience that relates to it, and it is nigh-instantly solved. (Just as if you'd asked Si about a fact, and the Si individual immediately recites the correct fact to you.)
Ni doesn't remember facts. It remembers how to solve "that kind of problem" in a very abstract way. As you might notice, putting "that kind of problem" into more concrete terms is very difficult.
Ne also chooses a perspective, not which is most evident and certainly not the most factual, but which seems to have the most potential to create something novel and/or something connected to a Ji concept (because these don't exist in a vacuum from the other functions). This is why NPs seem the most disconnected from reality or "absent-minded" or "dreamy" because it appears to be wishful thinking from the outside.
You are going to have more of an understanding of Ne than I do, but I don't think Ne chooses a perspective either.
Perhaps the difference is that you are judging dom and I am perceiving dom? You always end up choosing what you look at and how you look at it, perhaps? A blindness of Ni doms is that we don't really feel like we're "choosing" anything, that it's just there, but it probably looks like a "choice" to others.
Ni looks delusional too, but in the way where someone frames reality to be whatever it is they want it to be, with complete disregard for actual facts, so that no matter an outcome, it is what they predicted, because it can be framed that way.
Again, the notion of "whatever it is they want it to be" is off. This is how it looks to others, not to oneself. What you are experiencing perhaps from other Ni types is that they appear to want to change whatever it is you believe to be true, and you're immediate thought is likely, "But that's reality. You don't get to change that."
The reality is that they see a different reality than you do. They're looking at a different set of interconnections that you don't readily see, just as you see interconnections that Ni doms don't readily see.
Ni doms don't "choose" that perspective that they apply. It's what they see. It is their reality. AND they don't often realize that other people don't look at the world that way. Self-awareness for an INTJ is when we realize that no, we aren't that smart, but rather we simply don't look at the world the way everyone else seems to. Most people don't look at the world and see a complex system of cause-and-effect that can be altered in fundamental ways. One surprise for me was that what most people view as static, I see as entirely flexible and changeable. Conversely, what most people view as flexible and changeable I see as almost-immutable law, e.g., analogous to the laws of physics.
That's sort of like wishful thinking too, but the change is in the internal perspective, whereas with Ne, the possibilities are out there, and it's a matter of you being able to chase them, cultivate them, and create the change outside of yourself. For the Ne type, if the outcome is not turning out to be the potential initially seen, then they abandon and move on, whereas Ni types can stubbornly stick to it and insist it is what they said it would be.
Interesting perspective, here. I think what you are encountering is the different visions of the world, again, which see different kinds of things as mutable/immutable.
Quick and dirty function-theory version: To Ne/Si, the concrete is immutable, the abstract is mutable. To Ni/Se, the abstract is immutable, the concrete is mutable.
The Ni type seems less fickle because of this, but they are really abandoning an internal perspective in favor of a new one which shapes reality as they want it to be (or which gives them a sense of control of its outcomes),
More likely, circumstances changed (Se is mutable) which drew up a different Ni-experience-model-abstraction. To you, it looks like they just totally changed their mind, and don't even remember that you had just proven them wrong, for example. What really happened is that you believe there is only one context (Si is immutable), therefore the Ni dom just did a 180-degree logical reversal.
My ENFP ex and I had some really weird arguments along these lines early in our relationship, until after an explanation of mine she realized, "Hey, waitaminute! You changed context. You're not even talking about what I'm talking about now!" To which I replied, "Yes. Exactly. Why wasn't that obvious before?"
You might wonder how that fits into Ni being immutable. Ni is still immutable because the old context didn't just disappear. It still exists in abstract, but
doesn't apply now in the concrete instance. If the context switched back, you'd hear the same arguments as before the original switch. Further, there can be kind of a "chaos theory" kind of effect: a slight change in circumstances can produce a radically different conclusion even when maintaining the same Ni-understanding. Physics and math are full of things like this, where the math doesn't change at all, but a slightly different input value produces a significantly different result.