A baby can be killed by a front airbag, but has any adult driver been killed by an exploding airbag, due to sitting too close to the steering wheel?

Absolutely. This is a well-known cause of injury: a front seat driver or passenger who is not wearing his/her seatbelt and has a front-end collision, is at significant risk of death. Rapid acceleration of the airbag, in a crashing vehicle (rapid deceleration), causes a much bigger force than when a seatbelt is worn. The two forces are just too much. See comment for a link to problem/tweaking airbags.