"The "Conference2020" Joomla Group to be able to choose if their profile is visible to the public, only the "Conference2020" group, or private.
You can set User list to filter only users in some user group with the parameter called "Custom SQL Where", see examples at
http://docs.easy-profile.com/index.php/article/sql-custom-where
Or show only members that have some specific value in some fields, for example you can have a field called "Do you want the profile available in lists?" (alias:available_in_list, options:Yes/No). With custom SQL Where you can set a static filter like available_in_list LIKE "Yes"
How do I allow the custom visibility profile of "Conference2020" to be the privacy option only for the "Conference2020" group?"
You can't manage this natively, "visibility profile" is valid and equal for all users. A way to manage this is to create a specific plugin like
https://www.easy-profile.com/support/private-profiles-not-private.html#reply-26264
Unfortunately I does not understand well what's your goal because I think there is some misunderstanding about Joomla groups and ACLs, if "Conference2020" is a child of "Registered" then you will have that Conference2020 user group inherit ACL from registered user group, so for example all articles where the access is set to "Registered" will be accessible also for "Conference2020".