1. The price of gasoline at the pump lags the wellhead price of crude by around one month. Gasoline prices are dropping right now.
2. Diesel has more energy per gallon than gasoline, therefore a higher price for diesel is justified.
3. Your assumption that you get 42 gallons of gasoline from one barrel of oil is wrong. Crude oils from different areas are made up of different percentages of components. Some refined products are priced higher than gasoline, some lower. And yes, the margin profit on a refined barrel of oil is very small, lower than your guess of $0.67.
4. We live in a free market (at least until November 4). Prices of the same product vary from town-to-town, and vary from block-to-block. If a station owner wants to sell gasoline for $10/gallon or $1/gallon is his own business. I go to the station with the lowest price.