I'm going to answer the functional question first: if the principal thinks he's a good fit for grade one after extensive discussions with you and, critically, the preschool, then he is probably "mature" enough for this placement at this school. And wow, good for you that you have this thoughtful of an administrator, with not only willingness, but proactivity for thinking about your child's holistic development and growth mindset.
In the educational world, maturity can be defined as including some aspect of all of the qualities you've described, but really, most importantly, it's being able to manage the behavioral and self-management demands of an academic classroom, which the educators who know him best appear to believe he can. (BTW, his approach to conflict is perfectly appropriate at this stage of development, and actually quite sophisticated, since it requires being able to detach emotionally from the other person's behavior, and not allow someone else's bad choices to intrude on his inner life.)
