Explanation
No
No
Yes
a) When you assign a group to an application, only users in the group will have access. The assignment does not cascade to nested groups.
b) Tested in lab, existing owners will be replaced. Also direct assignment (resource owner) is path of least privilege. (replicated in test) c) Application setting 'visible to users' is set to No, then no users see this application on their My Apps portal and O365 launcher.
Reference
a) https://docs.microsoft.com/en-us/azure/active-directory/manage-apps/assign-user-or-group-access-portal b) maybe
https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-manage-groups c) https://docs.microsoft.com/en-us/azure/active-directory/manage-apps/application-properties#visible-to-users