Re: how many gallons can you drive after the fuel light comes on?
I think the tank holds over the 15.9 gallons. I suspect that when an accurate gas pump clicks off, you're full. But you can jam another gallon or so in until it comes out the filler. Not the best idea, but on the rare occasion that I find cheap gas I fill as much as I can. I,too, have put almost 17 gallons into an almost empty tank, usually with 390-400 miles driven.
As far as filling when it is cooler, I think the theory is that liquid volume expands and contracts with temperature. So theoretically cold gas is more dense than warm gas. I doubt it makes much difference - certainly not 15%. You can test that by putting a gallon of cold gas in a sealed container, drawing a line a the top level, and leaving it in the hot sun. You won't have noticeably more gas after several hours of heating. But you will have a heck of a bomb.
I subscribe to the not letting the tank get to empty out of prudence. But a good fuel filter should remove any sediment that gets into your gas tank. I still try to fill up at around 1/2 tank because that's the old price point for a full tank, and $30 is not as painful as $60+.