RE: How much in the tank after the gas light comes on?
There is no Serviceable fuel filter on most new cars. It is integral to the fuel pump (which is in-tank). Yes, allowing the tank to run low will encourage the collection of debris from the bottom of the tank. I have seen it many time. But the most impritant reason is the cooling. It is cooled by the fuel around it. Also, even though the tank is internally baffled, if it gets low enough you can slosh the fuel away from the pump, causing it to suck up air and the car will die.
Mitsubishi Master Tech