As for Diddy: a real, disinterested DOJ would say, "oh, you can testify against <people more famous than you>? Well, we might be able to cut you a deal."
Of course, if the DOJ is not interested in prosecuting those people, then no deal. If they think Diddy is already important enough and they can't let him slide, then no deal, or at least, a medium-stiff sentence.
Finally, two years in prison is not a picnic. See if you want to do it.