Joel is siphoning gas out of an abandoned car. 20 year old gas would be decomposed into varnish and would quickly destroy the engine of any vehicle in which it was used.
New graves dug out of hard dirt, the size of the hardened dirt is like plus 25%. Add in the size of the body, and it's like +50% for even a fairly shallow grave. Digging into hard dirt, adding a body, and patting it down flat with no leftover? It's why even shallow graves have a body shaped rounded mound over them.
Presumably they want to directly show the audience that he is out of ammunition and his situation now hopeless. Perry's rifle audibly clicks twice from trigger pulls when out of ammunition fighting the monsters. An AR of course would not work like that. After the last shot, the bolt would lock to the rear and the trigger could not be pulled. Even if he manually released the bolt forward, which you don't see him do, the trigger could be pulled once not twice.