1. I am not sure how the data internally is stored, but as per my experience I have seen or heard of any security issue with the data of any of our Clients.
2. No, the data once captured can't be deleted.
3. When the contract of the company expires, the data remains stored with Adobe Analytics. But, Clients would not have access to that unless they renew the account. Before the contract ends, client can get the data transferred to them.
4. Below is an article on data collection in Analytics: