The serpentine belt was due 20,000 miles, no wonder it snapped.
The airbag light ON could be by two main reasons: the battery is low and/or the airbag hardness went bad, something that Ford recognized it happens with the year of your car. The replacement part cost less than 200 bucks.
The alternator always will show you its failure when you drive for long periods, the battery won't recharge and your car will die in the middle of the way.
If not the battery or the alternator, check the cable terminals, clean them. Inspect if you have bad wires.
When you said that the battery is dead or almost dead each time you turn the car On, the problem must be a power leak somewhere. Check if bulbs come off completely when you leave the car, check if no devices are connected to the car outlets all night long, even can be a security alarm system pulling more power than the calibrated minimum.