When a user leaves and the AD account is disabled, we cant access their Inbox/Calendar if previously granted. Can we change this so that we can?

You can only access if you delete the user, run exchange cleanup agent and then associate the deleted account to another user.

Hi, You need to select Associated External Account on Self mailbox permission. Go to disabled users properties -> Exchange Advance -> Mailbox Rights -> Select Associated External Account permission of Self. Wait for the replication to get it applied on user then who have rights on that mailbox they can access.