If Don Copal was taking orders from Hector why was he demanding Manny get premium clients or risk being fired? He must have known Manny had no means of doing so.
It would be reasonable to assume Copal had no reason not to be hard on Manny for poor work, even if he knew there was no way for him to do better. Anything less than his usual hardass behavior would be suspicious. Alternately, he wanted an excuse to fire the guy.
Burning beavers (in't it a great name for a band?) are able to swim in a river of tar without setting it on fire.
Being on fire is what allows them to swim in the tar - you put out their flames, they sink instantly. Presumably it's not hot enough to actually light the tar.
In the conclusion of the game, Chepito decides to walk himself to the afterlife, heading into the Number 9 Tunnel before Manny returns to El Marrow. Considering the game ends with Manny boarding the Number 9 in less than a day from then, the train travels 4 years in four minutes, and Chepito walks pretty slow, feel free to draw a conclusion as to what could happen to him.
You completely missed the point. Of the ENTIRE game. The Number Nine takes four minutes to complete the journey from El Marrow to the tunnel. Walking from El Marrow takes four years (as demonstrated by Manny's journey), then it's just a formality of entering the tunnel. Chepito could be gone the second he enters the tunnel and he might not even go to the same afterlife as the train. No-one knows.