No you don't! You can buy one volume, or case on a 'one-off' basis.
If you renew your subscription, you can continue to have access to law reports from the previous year. But you have to renew your annual subscription to be able to do this.
Yes, this depends on how many licences you have paid for. If you have paid for more than one, then you can have multiple users using the account at the same time.