If my primary region goes down, my secondary region remains available, but I assume it will be in read-only mode: that is, my single read replica isn’t elected as a leader (is this assumption correct?).
Yep, that is correct. Keep in mind though, if you’re running the flex implementation, you need 3 nodes running within your primary region for HA.
Is there a way to configure my cluster to automatically fail over to my backup region, e.g. by running a second read replica in the backup region, so there are at least theoretically enough members to create a HA cluster?
We do not currently support automatic regional failovers. In the event you need to perform a regional failover on a Nomad setup, you can reference: High Availability & Global Replication · Fly Docs