Drupal commenting system vs Disqus
Regarding question 1, there are several options to do that, for instance OpenID Selector.

Question 2: Yes. I don't know all of Discus' features so I cannot answer this for each and every one of them. However I do know that there are Drupal modules for comment notification, for instance Comment Notify.

Questions 3-5: those are quite specific, I think you'd better ask them in the issue queue of the Drupal Disqus module.