Does removing a user from Analytics also delete all their associated Workspaces/dashboards that they have shared with others within the organization?
Yes, however you will have an opportunity to transfer all of those user's content to someone else before you delete them. If you ignore that warning asking to transfer their stuff, I'm afraid there isn't really a way to retrieve those reports.
I don't think so. However, it's recommended that we transfer all the assets/components to another user before deleting a user.