Well, for one, you can't do offensive/defensive subs for an entire half. You need dead balls after every possession.
Secondly, Coleman was getting abused by their interior guys and giving up as many points as he was getting. As it turned out, and JB pointed this out, our other bigs didn't exactly shut their guys down either. JB plays percentages. Coleman was getting put-backs and garbage baskets, not getting offensive sets run for him. So his ability to impact the game offensively was purely a matter of chance. He could have played 20 more minutes and not scored, it depended on rebounds being available, etc. But his horribleness on defense was consistent and so it was going to be there on every possession. A guy who plays percentages like JB, is not going to take that trade.
As for leaving Ennis in at the end. I'm sorry but my response is "duh." Ennis is a proven winner (watch his HS games especially crunch time in national games), and like MCW, is a guy who can languish for a whole game then make the game winning play. You want guys like that out there. It's not stats or anything else, it's putting the guy who is capable of winning you the game on the floor at winning time.