Can Skullcrack's damage be prevented?
As you quote in rule 608.2c, you resolve a spell or ability, effects in different sentences are resolved consecutively. When Skullcrack resolves, first it creates a continuous effect that stops players from gaining life for the rest of the turn. Then it creates a continuous effect that makes damage unpreventable for the rest of the turn. Finally, it deals the damage, which cannot be prevented because the effect stopping prevention already exists.