There are a number of things that need to be defined such as "defenseless" player. A defenseless player is one who is not in the game. He's on the sideline with his helmet off. If someone goes up and plows him over, he's defenseless. If you're on the playing field, you're not defenseless. You are always in pursuit whether you are two feet behind or twenty yards and should be able to be blocked. Therefore you need to be alert at all times since you are on the playing field. This is not even about the Bell hit anymore, it's about football in general. To me, there is no such thing as a defenseless player when you are on the playing field. The closest thing would be a referee I suppose, and it's amazing you don't see more of them get seriously injured.
Nobody wants to see anybody get hurt, but it's part of the game. If the physicality is continually changed, the game will change. You take an inherent risk when you play football. You should expect to get your bell rung at some point (no pun intended). You take an inherent risk when you pretty much do anything in life and you can find statistics to pretty much back that up - driving a car, getting struck by lightening, putting change in a vending machine, etc.
Helmet to helmet? Once again, we need a clear definition. I say if you leave the ground with both feet in the air (i.e. pull a superman and clearly hit with the helmet, therefore showing the intent), throw the flag. If you keep your feet on the ground, no penalty. So many times helmet to helmet can be called simply because the runner ducks his head. Remember the hit on Gabbert a few years back? Yes, it was helmet to helmet, but also inevitable.
Someone mentioned reviewing a play where a flag was thrown. Maybe that could work too.