No, core motivations are really easy to discern, it's simply about what you want in life, based of off what you fear, nothing more and nothing less.
Core motivations are desires. They are more or less based off fears depending upon one's level of development. Fear drives us to pursue these desires in more destructive ways, but fear is not necessarily present. When fear (anxiety, really) is entirely absent, then we are spiritually and psychologically free to move on to our type's point of integration in the pursuit of desires.
Reading descriptions only muddles the process because of the method in which descriptions are written; most descriptions are written without anything in the way of tested or external information. Most people who write the descriptions base them off simple axioms and rational deduce what the type is suppose to be, not what it is.
However, since you're partly addressing me, I have been quoting Riso who reportedly spent much time riding buses and taking copious notes of the people seated around him. This statement also applies to much of what you wrote just below this.
Because of this type descriptions read more like stereotyping horoscopes; this is why INFP descriptions make them look like fluffy bunnies, it makes INTP's look like socially inept nerds, etc. If typings of every sort are to ever be taken seriously in the real world, then they need to be tested much like a science before actual descriptions are written. For example, several groups of people are split up based upon a certain personality trait (fear/motivation, function usage), then observation is done in order to draw any general conclusions concerning similarities within their behavior. Once that information is carefully collected, only then can general descriptions be written. That's what needs to be done to the enneagram (and other types categories), otherwise relying on the descriptions alon is nonsense.
I certainly agree with the idea of core or key motivations of types, although it seems too rigid and archetypal to be applied in reality. All other motives reduce to a single one. I'm just saying that one cannot know their own key motivation without reading an Enneagram book and believing what is written there. A typical criticism of this idea would ask why only one key motivation and not two or a dozen. Why all the psychological reductionism, hasn't psychology grown beyond Freud?
Then what is the enneagram about?
Also, what makes you think that Elfboy, a guy who fearlessly argues against all his classmates, demonstrates excellent Te usage, and doesn't tak shit from others, is somehow one of the most introverted types (INFP 5w4) in existence?
Elfboy described that as one phase out of many. If you look back at his informative bio, there were various periods which he represented by some key behaviors. He vaguely mentioned some motivations but none of them match up with key motivations. Anger was one such motivation. Being a hero to others was another motivation.
There were five distinctly spelled-out phases to his childhood into adulthood. 1. The defender/protector/hero phase; 2. being picked on by bullies and retreating into a shell/confusion/caution phase; 3. an anger phase (although he was not aware of being angry), learning to defend himself primarily verbally, as you pointed out; 4. another quiet phase, not so much socially withdrawn this time but more of a self-confident charmer; 5. college, and drawing on his 3-ish ability to appear as one ennea-type while being another.
If I were forced to abandon the idea that he is a 6w5 I would have to go with 3, but with the wing type unknown. 6w5 still fits him better than any other type, to my mind. But not the 8 who willfully seeks out conflict, whereas he seemingly only attracted it at phase 3 as a kind of magnet for trouble.
I often use the "unknown-wing" to eliminate the wing's core type as a possibility.
That being said, I haven't denied that much of what Elfboy writes on himself comes across like the 8, although, among other things, he was rather too quiet and sensitive a child for that. Self-describing as a charming adult comes across as a higher-level 6, far far more than 8 or 3. Three's become charming as a manipulative ploy. Sixes simply want to make a good impression for various, more healthy reasons, mostly I think simply because they ENJOY being around other people.