Excellent analysis, Salmon and Mashed Potatoes!
You're right that using only the stats from last season is taking the talent of Big Baby completely out of the true context of his worth. The stats are one of the tools to use to analyse a player, but only one of many.
You pointed out, I believe, how getting more minutes improves a good players play and I have noticed that, on this excellent Celtics team, Baby has had about equal minutes to KG so far this preseason and has put up better numbers and better defense. As much as it hurts to point this out because I love KG, Baby is playing better.
Doc would not have Baby coming out right behind KG if he didn't see the excellent value he has in BBD.
What is obvious to most of us here is that Baby is really good, improving, able to improve (did you see all those jumpers last night?) and will, inevitably, IMO, be a starter for a very good team one day, and maybe sooner than we think.
All of the arguments for trade leave out major points, especially the fact that KG may very well get injured during the season and Doc will then be having BBD as his starter.