I've seen a theory that the mistakes Pemulis makes are intentional, and signal that he isn't as smart as he thinks and that he doesn't really have everything in control. I'm not sure; clearly DFW had some math aptitude but these also could have been honest mistakes. Presumably it would have been harder for editors to check these things in the 90s. The probability error seems harder to explain and likely just a mistak…
Vast majority of character's are well explained and the answers are all in the novel, just need to look at their childhood and upbringing.