Why is it important that the first and last chapters take place in the same location?
Because it is a neat, symmetrical way to round off the novel.
Partly right. The symmetry is not just to make things neat, though. It helps us understand that what happened to Lennie was inevitable.
What is the significance of the killing of Candy's dog?
It hints that George will kill Lennie.
Yes. Candy's dog was shot humanely to prevent him suffering any more. Lennie is shot humanely to prevent him suffering - and death - at the hands of Curley. Candy later regrets that he didn't have the strength of mind to…