Question 2: About Sirius, I don't know. We just don't have enough information.
I think he's dead, but I wouldn't bet the ranch.

Questions 1 and 3 go together. You can't have it both ways.
Either Snape is good and Dumbledore is alive, or Snape is bad and Dumbledore is dead.
The one thing you can't have is a good Snape and a dead Dumbledore.