Is there a way to see a list of all of the registered users on an instance?

afaik no (not yet?), you could look them up in the database

Are all Lemmy instances openly federated unless they are on the block list?

if your instance uses open federation, yes. it then basically federates with all other "open" instances and those who have yours on their allow-list (and which are not on your block-list).

if your instance uses an allow-list, only allowed instances could federate, no need for blocking here.