With regard to the quality of the fuel coming out of the pumps... it doesn't matter what logos are/are not shown on the delivery trucks, and it doesn't matter which specific refinery is filling the delivery trucks. There are multiple supplier and delivery contracts going on all the time, and they do change.
What matters is the fuel that's actually pumped into consumers' vehicles. The only way to prove what THAT is, is to have it tested. This testing is done randomly and regularly. Also the accuracy of the pumps is tested regularly (volume indicated vs volume delivered).
Violators are heavily fined.
Consumers will still occasionally get a bad tank of gas (water, sediment, etc) but it is extremely rare. Fuel is filtered (or at the very least strained) at every step of the process: from the refinery into the trucks, from the trucks into the retail storage tanks, and from the retail tanks into your vehicle's fuel tank. Gas stations that are not busy (slow inventory rotation) may have more issues than busy stations.
The point where the additive cocktail is added may be at different points -- when filling the tanker truck at the refinery, or when the truck unloads into the retail storage tank. But again what matters is what's coming out of the retail pump nozzles.
This is all based on my understanding of the industry. Please correct me if this is wrong.