1. set up a read-only slave database? The closest solution I could find is Hot Standby but the slave would not be accessible until after a failover.
Hot Standby will give you a standby database that is accessible, but in read-only mode. This sounds like what you're looking for.
2. temporary convert a read-only slave in read-write mode for testing read/write workloads? Currently in Oracle, we can temporary open our read-only standby database in read-write mode to occasionally test our read-write workloads. We would stop the log apply on the standby database, convert the read-only database to read-write, perform our read/write test, discard all the changes after testing and reopen and resync the standby database in read-only mode. Is there a similar feature in PostgreSQL or are there ways to achieve something close to our needs?
No, you can't do this with postgres natively.
You could snapshot your filesystem before opening it and then roll back to that snapshot, or something like that, but you cannot do it with just PostgreSQL functionality.
