Other mistake: After Pinball douses "Cochise" with flammable liquid, the match he throws extinguishes before it leaves his hand. (00:22:45)
Other mistake: When the plane is at Carson City for exchange of prisoners, the guards have jackets with 'Dept of Prisons' printed on the back, but there is a bus with 'Bureau of Prisons' written on the side. (00:35:50)
Other mistake: Mykelti Williamson, who plays Baby-O, is listed in the beginning credits as Myelti Williamson.
Chosen answer: Zero. As you said, he was attacked and there are witnesses that he tried to avoid the fight and the killings were in self-defense. It is an extremely weak plot hammer to get Poe onto a plane full of criminals. It's foolish as well. The writers could have had Poe framed for a crime then exonerated and put in the same situation much more believably.
Grumpy Scot
It's in Alabama. People are put in prison here for much less.
First, Poe is a federal prisoner, not subject to State laws or legal procedures. Secondly, he is not in Alabama. During a conversation with Billy Bedlam we hear that he is incarcerated in the "Q" - prison slang for San Quentin in California. It makes you wonder why a Federal prisoner is in a State prison, but that's another type of mistake.
Would it really be considered self-defense, though? After he beat the guys to the ground he could have just stopped and walked away, but he didn't. He kept beating them until they died.
He is defending his wife against two armed assailants, and use of lethal force is allowable. No DA in the United States would even think about pressing charges, knowing full well a grand jury would throw them out in a second.