You'd be amazed how many people learn the hard way that they first need to head to a separate store to buy a gas container, then head to the gas station. Most places could care less about keeping a container on premises because it is both a rare situation these days, and storage is at a premium in most stores. If you saw the back room or manager's office, you would see the floor-to-ceiling stacks of inventory (usually drinks or tobacco products).

As it's a federal regulation, it would need to be the feds pursuing it. It's highly unlikely they'd come after the individual. They would be more likely to pursue the gas station.

Presumably, you buy a grade other than the E15 that doesn't have the "minimum 4 gallons" restriction.

Around here stations have a purpose-built plastic bag that seals after filling up and fits in the car's fuel intake. It holds a few gallons, but I'm not sure how many.