Masonobeans, enotes.com policy is that you may only ask one question per posting, so I've narrowed your 3 questions down into one. I hope this helps.
In "Poison", Dahl begins hooking his reader with the presence of the extremely venomous snake - the krait. We, the readers, turn the pages and want to know what's going to happen: will Harry Pope get bitten by the snake? Will he die? Will the doctor and Timber save him or will they too be bitten by the krait? Readers ask themselves questions such as these all throughout the story. Harry's real sense of fear (he's petrified) and his unwillingness to move at all while waiting for rescue, add to the mood of the story and keep us wanting to know what will happen.
The krait itself is an external conflict (Harry Pope v. the venomous snake). However, the krait is also a symbol for the ideas of racisim presented when Harry Pope begins hurling racial slurs at the good doctor when they discover that there is no snake in the bed. Another type of poison could be fear - fear paralyzes some people, much like a poison could.
Internal conflict present is Harry's state of mind, his fear, and his racist remarks. He must really believe those racist things if he's saying them outloud.