Okay so I finally got to listen to the episode and can easily bring some prison/crime expertise to this thread and what was discussed on the show. In regards to the sentence Byrne received, I'm assuming the term murder is being used generally to describe his crime, which is most likely negligent homicide/involuntary manslaughter which carries a sentence of 6 months to a decade on average. I'm guessing it was like in Con Air where a fight broke out between two parties, in this case Byre and the guy sleeping with his wife, and in the course of the fight the other guy died by accident, like he fell and cracked his head on something. But rather than put that time and effort into explaining why he was behind bars. the writer just went with murder which in turn just creates more questions.
As for his drawing setup in his cell, that is a non-plausible thing as depending on the state, most prison cells aren't bigger than a small storage unit, usually 6' x 10'. Some newer facilities are going with a dorm style unit, called pods, which house around 6 inmates, and those are much larger in size. But that doesn't mean one guy can prop his shit up and just take over space, that will get his ass kicked in no time flat. Also, an inmate can make money while in prison from a personal business venture, they just can't make money off of their crime. A recent example was when OJ Simpson wrote his book If I Did It a couple years back that was very controversial, but he was able to write it and make money off of it as he was acquitted in his criminal trial. Tex Watson of the Manson family tried to do a similar thing at my facility a few years back with both a manuscript and I think some paintings, and that shit got shut down quickly because it was directly linked to his crimes, he also received some additional charges if I recall because of the back channel ways that he was trying to do it. So for a guy like Byrne to just be writing a comic book series and having it sold, that's completely fine, I mean at my facility there are a few guys who have self published books or have sent art out for sale, and they're able to as it doesn't pertain to their crimes.