Replaced the fuel pump in an old Altima a few days ago. Access through a panel under the rear seat. Removed panel and the pump, then drained 7 gallons of 8 year old gas from the tank (the car has been sitting 8 years) in less than 10 minutes with a transfer pump. Gas smelled a little odd, so I'd say it was bad, but still, gas is gas. Poured it into the half full tank of my driver, and it ran just fine.
1. So yes $600 seems excessive. Why the need to drop the tank? If you can't siphon it out the filler, or you don't have access from the top, you could command the fuel pump on and collect it at the fuel rail.
2. What exactly was bad about the gas? I assume it wasn't age. And what about it would cause the car not to run?