With respect to the walker that was evidently stabbed in the neck but dropped "dead" anyway, it is possible (you couldn't really tell from the camera angle) that the knife stabbed upward into the brain stem, so destroying the medulla oblongata and thus neutralizing the walker.
When Polly opens the door to leave the room after arranging to meet with Rick to negotiate a trade "in 10 minutes", there are two walkers she has to get past. She stabs them both, but only the first one gets it in the head. She clearly stabs the second one in the neck rather than the head and it drops to the floor. This would not work on a walker.
When Carol lights the cigarette and goes to throw it in the room with Negan's men to burn them the first shot shows the entire floor and it is completely dry, but on the next shot the floor is soaked in gasoline.