I live in the Detroit / Ann Arbor area but I don't follow UoM football, however, at the beginning of the year the word was that JJ was not an exceptional passer. As the year went on he definitely showed that he could pass; not that he was a superstar but that he was better than adequate. I believe his arm was given credit for a few wins, if not more.
Your observations and comments about JJ coincide with what the folks here say about JJ.
I would not draft him to be my starting QB because he hasn't demonstrated top tier talent in throwing the ball.
Maybe his skill has been hidden because of Michigan's strong running game, or maybe they have a strong running game because the coaches knew he didn't have the passing skills to be a top tier QB. Either way, I don't draft him.