1) To delete the payment history for the order you need to delete the order that its associated with. From the payment history screen click the select button to view the order or fro from the store admin menu click "Manage Orders" and there is a delete button there. Or from sql mgmt studio you could issue the following sql command "truncate table smith_payhist"
2) That message means that its trying to decrypt a null credit card number. Programming had fixed this issue some time ago and added logic to test for null before decrypting. What version of the cart are you running?
3) Status of "False" means the order was declined.
-Scott