Redis and postgres are very different tools serving different problem domains. If you're able to swap one out for the other, you might want to reëvaluate your architecture.
Not always. Both can be used as backends for persistent message queues since both have built-in pub/sub. If you do not have high throughput or latency requirements both are perfectly fine for this. If you already use PostgreSQL it is not always worth breaking out key value storage into Redis either.
Redis Pub/Sub is /not/ persistent. You can go the resque / sidekiq route and use a List and do blocking pops. But you have to make sure you have some persistence in place that you feel comfortable with (maybe a master / slave setup, with master handling all real queue load, and the slave doing AOF for recoverability).
