Should I turn off the airbag in the passenger seat to avoid being injured by it?

No, always lean back. You may increase your risk of air bag injury by chronicly leaning forward while seated.The best outcome comes when a rider travels leaning back into the seat, with the head no more than a few inches in front of the head rest. The driver's chest should remain at least 12in back from the wheel & its bag system. Proper riding habits give you protection from both belt & bag systems.
No. I would look at it more this way... If you're in an accident, the airbag is much more likely to prevent injury than anything, and is a great good. If it's going to cause injury because you have a medical condition or do not meet the height requirements, then you shouldn't be sitting there anyways. Children should not be in the front seat until they meet safety height standards.
No. In adults, air bag injuries are primarily caused by improper or lack of use of seatbelts. Always use both.