I know there is a long way between 6th grade boys (11-13 years old) and the NBA but I think Biz's lack of experience makes this applicable.
I coach a very competitive 6th grade AAU team that is full of kids that have all played basketball since they were 3 or 4 years old (my son included). Most of my players play year round and have played on 20-30 different rec league teams before they made the jump to AAU.
I have one kid though who, as of 1 year ago, had never played organized basketball before and had been more of a youth football type player. I knew this when I selected him for my team. Even though he is a man-child physically, many people (other coaches/parents/etc) questioned my selection of him early in the year since he was fouling everyone on D and was travelling or turning the ball over almost every time he touched the ball. I saw potential. A lot of work and development, but potential.
I awarded this player a roster spot and although it has only been 1 year, he is developing as I had hoped and, in certain areas, is one of the best on the team. His biggest challenge is his basketball IQ as most kids just know where to be and what to do in almost every situation. Things that are easy for most kids at this level, are sometimes challenging for him but he has the work ethic and desire to "get it". You can't teach size and natural ability. This kid has "it" and that can't be ignored. Regardless of how long it takes, I know he can be the best player on my team. That is worth the wait and worth the work it takes to get there.
How does this relate to Biz? Well, Biz is "new" at the game. Most NBA players have been playing since they could walk, not Biz. Most NBA players have been working towards that goal since they could walk, not Biz.
To say that he should excel at everything is fair but that isn't really realistic at this point. I 100% agree with the mindset of breaking things down and making it simpler for him to learn. By everyone's account, he has the desire and work ethic. I see nothing wrong at all with having him focus on specific areas for improvement.
Signing Jefferson and drafting Zellar have no negative implications for Biz. When he was drafted, I can't imagine Cho or anyone else thought he would be ready to start at this point of his career. That pick was/is about the player he can be at 25 years old. I personally still see him as the guy who could come in behind Jefferson as our starting center, and be that for almost a decade. What more can we really be wanting? Learning from a veteran like Jefferson is one of the best things to happen to Biz from a development standpoint. Don't give up on Biz too early.