Can one creature block multiple attacks?
No, one creature cannot be chosen to block multiple attackers (unless otherwise specified on the card, like Avatar of Hope). Each blocking creature may be assigned to one attacking creature. The rule, as you've pointed out, states (emphasis mine):


  509.1a The defending player chooses which creatures that he or she controls, if any, will block. The
  chosen creatures must be untapped. For each of the chosen creatures, the defending player
  chooses one creature for it to block that’s attacking him, her, or a planeswalker he or she
  controls.


The rule starts out by saying that the defender chooses all creatures that will block. Declaring all blockers at the same time is important. The defending player then assigns each blocker to one creature to block. Since all blockers are declared at the same time, it cannot be assigned to block one creature multiple times; each blocker is assigned to one attacker when blockers are declared.