[In response to question 2: the benefits of the proliferation of categorizations you mention in your last paragraph.]
Frankly, I don’t have a direct answer to this $64,000 question. The fact is that educators see differences between individual learners. But research to date is littered with inconsistencies; methodologies for prising out learner difference are often weakly supported or are proprietary; (cynically) academics promote their own differences in research that allow for career development more than pure theoretical and practical knowledge progression. Yet, many ideas are so intuitively right (i.e. supported not by empirical research but by lived experience) and motivating for teachers that abandoning them seems misplaced and reductionist.
Kozhevnikov and colleagues (2014) are a case in point. They desperately try to synthesise strands of research, but page 11 of their paper is basically an excellent argument against their own position. Willingham, Hughes, and Dobolyi (2015) continue Coffield et al’s discourse. But none of this helps answer your question.
An EdD thesis into this matter would fall into Coffield’s trap. But following Hattie’s insistence on effect sizes as the “Holy grail” (Hattie, 2009, cover page) of education is also, in my opinion, problematic because curricula that aim to reproduce historical effects may fail to grasp the complexity underlying learning and reduce potential to what is desirable by a few. At best, Hattie and Dweck (using two representatives) offer educators insights into the maze of learning, into what is possible, not what is probable.
Perhaps I can offer a metaphor of the umbrella. The teacher is the middle spoke, the learners everyone under the umbrella. What happens under the umbrella is unknown until the point of learning. The number of spokes represents the number of teacher interventions available. The more spokes, the more possibility. In all probability, very few interventions are needed. However, the range and depth available to a teacher are increased through study of any and all ideas that have a bearing on the educational environment.
Hattie, J. (2009). Visible learning: a synthesis of over 800 meta-analyses relating to achievement. Abingdon: Routledge.
Kozhevnikov, M., Evans, C., & Kosslyn, S. M. (2014). Cognitive Style as Environmentally Sensitive Individual Differences in Cognition: A Modern Synthesis and Applications in Education, Business, and Management. Psychological Science in the Public Interest, 15(1), 3–33. doi:10.1177/1529100614525555
Willingham, D. T., Hughes, E. M., & Dobolyi, D. G. (2015). The Scientific Status of Learning Styles Theories. Teaching of Psychology, 42(3), 266–271. doi:10.1177/0098628315589505