That `sys::TransactionIsolation` doc is outdated, EdgeDB only supports the `Serializable` transaction isolation. See related discussion [1]
At this point EdgeDB supports Postgres HA passively, i.e. it will react to a failover event in your cluster via one of the documented mechanisms. Support for "active" cluster management is a planned feature too.
Finally, EdgeDB server itself is fully stateless and you can run multiple instances of it in front of the same Postgres cluster. Admittedly, we need to document this better.