I agree it's unlikely that Gregory comes back, but it's not crazy. It's possible to realize that a solid insurance policy + no likely consequences to your draft status makes one last year of college football (an opportunity you will never ever have again) worth the risk.
Who wants to play for the raiders? I think the defense is realizing how good they can be now, and what they can be next year - with Rose, LeRoy, CJax, Moss returning - that's a ton of talent. Plus, everyone knows Shaq got rich in college.