I hope I can help out here and give you some pointers on solving yous challenges. First I would suggest to create a group with all you users so you can use this to grant permissions.
For the shared calendar I would create a shared mailbox (resource) and disable the auto meeting responses and add 1 or 2 users as delegate to this mailbox. This will allow you users to send meetings to this shared mailbox and the delegates can manage them. If you want your users to directly add meetings, one of the delegates can add read/write permissions (for the group that was created earlier) to the calendar of the shared mailbox and allow all users to open the calendar and add their appointments.
For the calendars of the CEO and President everyone by default can already see "free busy" information. You can control the level of that via the calendar permissions. This will allow users to see availability when booking meetings. Some clients will be able to open the calendar with these permissions (Windows Outlook). Other clients will need read permissions at a minimum to open the calendar as shared calendar. For their personal secretaries I would add them as delegate on the mailbox so they can manage their mailboxes on their behalf.
Here is a documentation page that might be helpful to learn more about permissions and delegation: AWS Docs
Kind regards, Robin
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