I doubt they'd need to pay this guy a huge NIL deal. I don't think anyone else will. And even if it is relatively modest by P5 standards his Nebraska NIL deal would probably a lot more than he'd get in FCS.
Rhule would be doing HH a favor - he could go back to TE
I don't think Rhule would want to throw Kaelin to the wolves if, say, Raiola got hurt in game two. You risk ruining him.
You also have the issue of not having enough real arms to properly run practices that can develop some of these young receivers, The young receivers need to be catching passes.