1. We'll ship a major version without classic queue mirroring at some point next year. The last version still containing this feature will be supported for at least a few months after that happens.
2. No, quorum-based systems require an odd number of members.
3. Some classic queue features like DLX and TTL are coming to quorum queues. However, priorities are unlikely to be implemented in quorum queues. You can achieve the same functionality by using multiple queues, which is exactly what happens when you use priorities in classic queues (messages are internally stored in multiple queues).
Best,