Typically, you will have at least two that are candidates. That way, one can take over—if the other crashes, needs to be upgraded, etc.
(For best results, min_machines_running = 2, otherwise the second won’t have all the latest, 
.)
Your future nodes in Chicago or Sydney (or wherever) will not be candidates, though, and hence will never become primary.
No, only one at a time. The lease machinery is what enforces that.
Newly started candidates will ask to be named primary automatically (due to promote: true), so that may have confused the issue.
Try turning off auto-stop temporarily and then retry.
(You should also see various chatter about primary lease acquired, etc., in the logs.)