Excellent breakdown. You brought to light a conundrum that defenses face vs. Hill, especially when he comes out in an empty backfield set.
You have a 240 lb power-running QB in the backfield who clocks at a 4.4 40. But even in say, 21 personnel, having Kamara at RB often draws a nickel set because he can run routes and catch the ball like a WR (you don't want a LB on him). So you're already replacing LBs with DBs to play matchup outside, while taking LBs and DL off the field against a 240 lb QB who breaks tackles like twigs.
So now, are you gonna rush 4 and play zone, giving Hill rush lanes and a head of steam? Or are you going to go cover 0, bring heat to close off rush lanes and stop him before he gets started... but also have the rest of your defense playing man, turning their back to the QB, and not in position to rally if he escapes.
Having Hill on the field, at QB, makes it really difficult to properly group your defensive personnel.