Design a complex billing system that tracks every state change for audit purposes using Event Sourcing and Command Query Responsibility Segregation (CQRS). 1. Detail the structure of an 'Event' in this context. What metadata is essential? 2. Explain how you would handle 'Event Versioning' as the billing logic evolves over time and existing events in the store cannot be changed. 3. Describe the process of building the 'Read Model' from the stream of events. How do you handle projections that need to combine data from multiple streams? 4. Identify the potential pitfalls of using Event Sourcing, specifically regarding 'Event Schema Evolution' and the complexity of debugging temporal issues. 5. Provide a pseudo-code algorithm for replaying events to rebuild the state of a specific billing account.