Most mock drafts are posted for entertainment value and are put together by people who have little or no knowledge of the subject. Thus I wouldn’t put too much stock in them. Like you say, the best way to get an idea of where a player might go is to declare for the draft but not hire an agent. I think there is a good chance he will do this.
The risk of staying is that a number of players over the years have seen their stock fall by not leaving. When dealing with underclassmen, there is a built-in expectation of improvement each year. If he does not improve by a significant amount, his stock could fall. Also his value in Europe – if he doesn’t end up on an NBA roster – might be at an all-time high right now due to all the publicity of the Duke game.
One other factor of which I have no idea. Does his family need the money? As for finishing college quickly, I’m sure he will get his degree long before he needs it for anything whether he stays or goes.
I’m sure it’s a tough decision for him.