Here is some D-League info on alumni (Terrence Jones is prolly best comp of those):
http://www.nba.com/dleague/tracker/
D-League has seen a drastic increase from what it used to be (and the D-League I think you are thinking of)
You keep bringing up Donte Greene. He was a first rounder and got guaranteed money. Chad Ford isn't a NBA scout, so a better comparison would be to where NBA Assistant GM's had him, since that is the projection you are working on for Grant.
I would say they saw Greene more in the 20-25 range like Grant.
There are no European draft prospects that NBA teams don't know about at this stage (though surely there are some we don't know about).
If he hurts his back in draft workouts, that hurts his stock. But, if he aggravates in draft workouts or aggravates it during the college season, not sure there is much of a difference. Plus, the player has a lot of control over workouts and when they compete.
If Grant is truly hearing 20-25, he has a hard decision. I am not saying he should choose one way or the other or what I would do in the situation. I am merely trying to get you to see that the NBDL is much improved and isn't the death knell it used to be.