Is there any scientific study showing the development of fluency in relation to the personality traits of a learner in the acquisition of English as a second language?
Yes, most definitely. Personalities affect language acquisition as well as fluency. Reversely, languages influence personalities, too, which is also an interesting topic to study.
Yes...Different learners have different learning styles. That is why it's important to identify the learner's learning style in order to succeed in the teaching and learning process.